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>
diff --git a/arch/mips/ath79/Kconfig b/arch/mips/ath79/Kconfig
index f44feee..cffdc8e 100644
--- a/arch/mips/ath79/Kconfig
+++ b/arch/mips/ath79/Kconfig
@@ -88,6 +88,10 @@
 	select PCI_AR724X if PCI
 	def_bool n
 
+config SOC_QCA955X
+	select USB_ARCH_HAS_EHCI
+	def_bool n
+
 config PCI_AR724X
 	def_bool n