mvebu: add support for Buffalo LinkStation LS421DE

mvebu: add support for Buffalo LinkStation LS421DE

Buffalo LinkStation LS421DE is a dual bay NAS, based on Marvell Armada 370

Hardware:
   SoC:         Marvell Armada 88F6707-A1
   CPU:         Cortex-A9 1200 MHz, 1 core
   Flash:       SPI-NOR 1 MiB, NAND 512 MiB
   RAM:         DDR3 512 MiB
   Ethernet:    1x 10/100/1000 Mbps
   USB:         1x 2.0, 1x 3.0
   SATA:        2x 3.0 Gbps
   LEDs/Input : 5x / 2x (1x button, 1x slide-switch)
   RTC:         Ricoh RS5C372A, I2C, no battery

Flash instruction (UART+TFTP):
  1. Downgrade the OEM firmware to 1.34 version (BUFFALO_BOOTVER=0.13)
  2. Remove any hard drive from inside the bays.
  3. Boot the Openwrt initramfs image using the U-Boot serial console:
         tftpboot 0x1200000 buffalo_ls421de-initramfs-kernel.bin
         bootm 0x1200000
  4. Flash the sysupgrade image using the Openwrt console:
         sysupgrade -n buffalo_ls421de-squashfs-sysupgrade.bin
  5. Wait until it finish, the device will reboot with Openwrt installed
     on the NAND flash.

Note:
  - Device shuting down doesn't work, even if the power slide switch is
    used. We must first, via MDIO, set the unused LED2 at the ethernet
    phy0 to off state. Reboot works ok.

Signed-off-by: Daniel González Cabanelas <dgcbueu@gmail.com>
Reviewed-by: Tomasz Maciej Nowak <tomek_n@o2.pl>