mips: ath79: Add AR934x support

Add support for the Atheros AR934x WiSoCs. This patchs adds complete
system init, including PLL and DRAM init, both of which happen from
full C environment, since the AR934x has proper SRAM.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Wills Wang <wills.wang@live.com>