ath79: add support for Atheros DB120 reference board

ath79: add support for Atheros DB120 reference board

Specifications:

SoC:    QCA9344
DRAM:   128Mb DDR2
Flash:  8Mb SPI-NOR, 128Mb NAND flash
Swtich: 4+1x 10/100Mbps via AR8229 switch (integrated into SoC),
        4+1x 10/100/1000Mbps via QCA8237 via GMII
WLAN:   AR9300 (2.4G+5G) + AR9340 (5G-only)
USB:    1x 2.0
UART:   standard QCA UART header
JTAG:   yes
Button: 1x reset
LEDs:   a lot
Slots:  2x mPCIe + 1x mini-PCI, but using them requires
        additional undocumented wiring changes.

Installation:
TBD, depending on pepe2k's u-boot mod.

Caveats:
AR8229 switch is not initialized properly yet.

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