MIPS: ath79: add SoC detection code for AR934X

Also add 'soc_is_ar934[124x]' helper functions and a Kconfig
symbol for the AR934X SoCs.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Luis R. Rodriguez <mcgrof@qca.qualcomm.com>
Cc: linux-mips@linux-mips.org
Cc: mcgrof@infradead.org
Patchwork: https://patchwork.linux-mips.org/patch/3506/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>