ramips: add support for Sercomm S1500 devices

ramips: add support for Sercomm S1500 devices

This commit adds support for following wireless routers:
 - Beeline SmartBox PRO (Serсomm S1500 AWI)
 - WiFire S1500.NBN (Serсomm S1500 BUC)

This commit is based on this PR:
Link: https://github.com/openwrt/openwrt/pull/4770
 - Author: Maximilian Weinmann <x1@disroot.org>
The opening of this PR was agreed with author.

My changes:
- Sorting, minor changes and some movings between dts and dtsi
- Move leds to dts when possible
- Recipes for the factory image
- Update of the installation/recovery/return to stock guides
- Add reset GPIO for the pcie1

Common specification
--------------------
SoC:        MediaTek MT7621AT (880 MHz, 2 cores)
Switch:     MediaTek MT7530 (via SoC MT7621AT)
Wireless:   2.4 GHz, MT7602EN, b/g/n, 2x2
Wireless:   5 GHz, MT7612EN, a/n/ac, 2x2
Ethernet:   5 ports - 5×GbE (WAN, LAN1-4)
Mini PCIe:  via J2 on PCB, not soldered on the board
UART:       J4 -> GND[], TX, VCC(3.3V), RX
BootLoader: U-Boot SerComm/Mediatek

Beeline SmartBox PRO specification
----------------------------------
RAM (Nanya NT5CB128M16FP): 256 MiB
NAND-Flash (ESMT F59L2G81A): 256 MiB
USB ports: 2xUSB2.0
LEDs: Status (white), WPS (blue), 2g (white), 5g (white) + 10 LED Ethernet
Buttons: 2 button (reset, wps), 1 switch button (ROUT<->REP)
Power: 12 VDC, 1.5 A
PCB Sticker: 970AWI0QW00N256SMT Ver. 1.0
CSN: SG15********
MAC LAN: 94:4A:0C:**:**:**
Manufacturer's code: 0AWI0500QW1

WiFire S1500.NBN specification
------------------------------
RAM (Nanya NT5CC64M16GP): 128 MiB
NAND-Flash (ESMT F59L1G81MA): 128 MiB
USB ports: 1xUSB2.0
LEDs: Status (white), WPS (white), 2g (white), 5g (white) + 10 LED Ethernet
Buttons: 2 button (RESET, WPS)
Power: 12 VDC, 1.0 A
PCB Sticker: 970BUC0RW00N128SMT Ver. 1.0
CSN: MH16********
MAC WAN: E0:60:66:**:**:**
Manufacturer's code: 0BUC0500RW1

MAC address table (PRO)
-----------------------
use   address   source
LAN   *:23      factory 0x1000 (label)
WAN   *:24      factory $label +1
2g    *:23      factory $label
5g    *:25      factory $label +2

MAC addresses (NBN)
-------------------
use   address   source
LAN   *:0e      factory 0x1000
WAN   *:0f      LAN +1 (label)
2g    *:0f      LAN +1
5g    *:10      LAN +2

OEM easy installation
---------------------
1. Remove all dots from the factory image filename (except the dot
   before file extension)
2. Upload and update the firmware via the original web interface
3. Two options are possible after the reboot:
   a. OpenWrt - that's OK, the mission accomplished
   b. Stock firmware - install Stock firmware (to switch booflag from
      Sercomm0 to Sercomm1) and then OpenWrt factory image.

Return to Stock
---------------
1. Change the bootflag to Sercomm1 in OpenWrt CLI and then reboot:
   printf 1 | dd bs=1 seek=7 count=1 of=/dev/mtdblock2
   reboot
2. Install stock firmware via the web OEM firmware interface

Recovery
--------
Use sercomm-recovery tool.
Link: https://github.com/danitool/sercomm-recovery
Tested-by: Pavel Ivanov <pi635v@gmail.com>
Tested-by: Denis Myshaev <denis.myshaev@gmail.com>
Tested-by: Oleg Galeev <olegingaleev@gmail.com>
Tested-By: Ivan Pavlov <AuthorReflex@gmail.com>
Co-authored-by: Maximilian Weinmann <x1@disroot.org>
Signed-off-by: Mikhail Zhilkin <csharper2005@gmail.com>