ath79: add support for Atheros PB42 reference board

ath79: add support for Atheros PB42 reference board

Specifications:

SoC: AR7130
DRAM: 32Mb DDR1
Flash: 8Mb SPI-NOR
LAN: 4x 10/100Mbps via ADM6996 switch on MII
WAN: 1x 10/100Mbps via RMII
PCI: 2x mini-PCI
UART: standard QCA UART header
JTAG: yes
Button: 1x WPS, 1x reset
LEDs: 8x LEDs

Installation instructions:

The original FIS partitioning does not provide enough
space for the kernel, so the installation must start
with re-partitioning the flash from RedBoot.

The suggested partition table is the following:

RedBoot           0xBF000000  0xBF000000  0x00040000  0x00000000
kernel            0xBF040000  0x80060000  0x001F0000  0x80060000
rootfs            0xBF230000  0x00000000  0x005B0000  0x00000000
FIS directory     0xBF7E0000  0xBF7E0000  0x0000F000  0x00000000
RedBoot config    0xBF7EF000  0xBF7EF000  0x00001000  0x00000000

Which can be created with:
fis init
fis create -r 0x80060000 -e 0x80060000 -b 0xbf040000 -l 0x220000 kernel
fis create -b 0xbf260000 -l 0x580000 rootfs

The kernel and the rootfs must then be loaded from TFTP:

load -r -v -b 0x80060000 openwrt-ath79-generic-atheros_pb42-squashfs-kernel.bin
fis create kernel
load -r -v -b 0x80100000 openwrt-ath79-generic-atheros_pb42-squashfs-rootfs.bin
fis create rootfs

Once OpenWrt is installed, the normal sysupgrade method will work for
upgrades.

The MAC address is stored in the "boardconfig" partition, which is
automatically created.

Signed-off-by: Zoltan HERPAI <wigyori@uid0.hu>