MIPS: ath79: Add config symbol for the AR933X SoCs

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: Kathy Giori <kgiori@qca.qualcomm.com>
Cc: "Luis R.  Rodriguez" <rodrigue@qca.qualcomm.com>
Patchwork: https://patchwork.linux-mips.org/patch/2525/


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