kirkwood: add support for NETGEAR ReadyNAS Duo v2

kirkwood: add support for NETGEAR ReadyNAS Duo v2

NETGEAR ReadyNAS Duo v2 is a NAS based on Marvell kirkwood SoC.

Specification:
 - Processor Marvell 88F6282 (1.6 GHz)
 - 256MB RAM
 - 128MB NAND
 - 1x GBE LAN port (PHY: Marvell 88E1318)
 - 1x USB 2.0
 - 2x USB 3.0
 - 2x SATA
 - 3x button
 - 5x leds
 - serial on J5 connector accessible from rear panel
   (115200 8N1) (VCC,TX,RX,GND) (3V3 LOGIC!)

Installation by USB + serial:
  - Copy initramfs image to fat32 usb drive
  - Connect pendrive to USB 2.0 front socket
  - Connect serial console
  - Stop booting in u-boot
  - Do:
	usb reset
        setenv bootargs 'console=ttyS0,115200n8 earlyprintk'
        setenv bootcmd 'nand read.e 0x1200000 0x200000 0x600000;bootm 0x1200000'
        saveenv
	fatload usb 0:1 0x1200000 openwrt-kirkwood-netgear_readynas-duo-v2-initramfs-uImage
	bootm 0x1200000
  - copy sysupgrade image via ssh.
  - run sysupgrade

Installation by TFTP + serial:
  - Setup TFTP server and copy initramfs image
  - Connect serial console
  - Stop booting in u-boot
  - Do:
	setenv bootargs 'console=ttyS0,115200n8 earlyprintk'
	setenv bootcmd 'nand read.e 0x1200000 0x200000 0x600000;bootm 0x1200000'
	saveenv
	setenv serverip 192.168.1.1
	setenv ipaddr 192.168.1.2
	tftpboot 0x1200000 openwrt-kirkwood-netgear_readynas-duo-v2-initramfs-uImage
	bootm 0x1200000
  - copy sysupgrade image via ssh.
  - run sysupgrade

Known issues:
  - Power button and PHY INTn pin are connected to the same GPIO. It
    causes that every network restart button is pressed in system.
    As workaround, button is used as regular BTN_1.

For more info please look at file:
RND_5.3.13_WW.src/u-boot/board/mv_feroceon/mv_hal/usibootup/usibootup.c
from Netgear GPL sources.

Tested-by: Raylynn Knight <rayknight@me.com>
Tested-by: Lech Perczak <lech.perczak@gmail.com>
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>