kirkwood: add support for Zyxel NSA310S

kirkwood: add support for Zyxel NSA310S

Zyxel NSA310S is a NAS based on Marvell kirkwood SoC.

Specification:
 - Processor Marvell 88F6702 1 GHz
 - 256MB RAM
 - 128MB NAND
 - 1x GBE LAN port (PHY: Marvell 88E1318)
 - 2x USB 2.0
 - 1x SATA
 - 3x button
 - 7x leds
 - serial on J1 connector (115200 8N1) (GND-NOPIN-RX-TX-VCC)

Known issues:
 - no kernel module for RTC. [*]
 - buzzer (connected to MPP43) need to be drived by 1kHz signal
 - no kernel module for internal MCU connected via I2C[**]

[*]
Karoly Pocsi made simple, unofficial driver for HT1382.
It can be found here:
https://www.madadmin.com/zyxel-nsa320s-es-debian-linux-4-resz/

[**]
Karoly Pocsi found how CPU talk with MCU:
It is possible to query the MCU-controlled fan speed and temperature:
i2cget -y 0x0 0x0a 0x07
i2cget -y 0x0 0x0a 0x08
The first value (0x07) is the temperature in ° C, the second (0x08) is
the time in milliseconds to complete one fan revolution (rpm = 60,000 / value).
Info translated from:
https://www.madadmin.com/zyxel-nsa320s-es-debian-linux-4-resz/

Installation:

TFTP:
1. Run serial console and go to u-boot.
2. Copy u-boot via tftp and write to NAND:
	=> mw 0x0800000 0xffff 0x100000
	=> nand erase 0x0 100000
	=> setenv ipaddr 192.168.1.2
	=> setenv serverip 192.168.1.4
	=> tftp 0x0800000 nsa310s-u-boot.kwb
	=> nand write 0x0800000 0x0 0x100000
	=> reset
3. Run new u-Boot, repair bootcmd and restore MAC address from sticker
	=> setenv ethaddr AA:BB:CC:DD:EE:FF
	=> saveenv
4. Copy and run initramfs image
	=> setenv ipaddr 192.168.1.2
	=> setenv serverip 192.168.1.4
	=> tftpboot zyxel_nsa310s-initramfs-uImage
	=> bootm 0x800000
5. Download sysupgrade image and perform sysupgrade

USB:

1. Prepare usb fat32 drive with u-boot.kwb and initramfs image.
   Stick it to USB 2.0 port.
2. Run serial console and go to u-boot.
3. Copy u-boot from usb and write to NAND:
	=> mw 0x0800000 0xffff 0x100000
	=> nand erase 0x0 100000
	=> usb start
	=> fatload usb 0 0x0800000 u-boot.kwb
	=> nand write 0x0800000 0x0 0x100000
	=> reset
4. Run new u-Boot, repair bootcmd and restore MAC address from sticker
	=> setenv ethaddr AA:BB:CC:DD:EE:FF
	=> saveenv
5. Copy and run initramfs image:
	=> usb start
	=> fatload usb 0 0x0800000 initramfs-uImage
	=> bootm 0x800000
6. Download sysupgrade image and perform sysupgrade.

Based on work ThBexx <thomas.beckler@hotmail.com>
DTS based on dropped support in 0ebdf0c.

Tested-by: Lech Perczak <lech.perczak@gmail.com>
Reviewed-by: Lech Perczak <lech.perczak@gmail.com>
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
[NSA310s -> NSA310S in DEVICE_MODEL]
Signed-off-by: Petr Štetiar <ynezz@true.cz>