MIPS: ath79: add SoC detection code for the QCA955X SoCs

Also add 'soc_is_qca955[68x]' helper functions
and a Kconfig symbol for the SoC family.

Cc: Rodriguez, Luis <rodrigue@qca.qualcomm.com>
Cc: Giori, Kathy <kgiori@qca.qualcomm.com>
Cc: QCA Linux Team <qca-linux-team@qca.qualcomm.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4943/


Signed-off-by: John Crispin <blogic@openwrt.org>