arm64, thunder: Add Kconfig option for Cavium Thunder SoC Family

This introduces ARCH_THUNDER to enable soc specific drivers and dtb
files.
Signed-off-by: Radha Mohan Chintakuntla <rchintakuntla@cavium.com>
Signed-off-by: Robert Richter <rrichter@cavium.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

arch/arm64/Kconfig

@@ -134,6 +134,11 @@ source "kernel/Kconfig.freezer"
 
 menu "Platform selection"
 
+config ARCH_THUNDER
+	bool "Cavium Inc. Thunder SoC Family"
+	help
+	  This enables support for Cavium's Thunder Family of SoCs.
+
 config ARCH_VEXPRESS
 	bool "ARMv8 software model (Versatile Express)"
 	select ARCH_REQUIRE_GPIOLIB

arch/arm64/boot/dts/Makefile

+dtb-$(CONFIG_ARCH_THUNDER) += thunder-88xx.dtb
 dtb-$(CONFIG_ARCH_VEXPRESS) += rtsm_ve-aemv8a.dtb foundation-v8.dtb
 dtb-$(CONFIG_ARCH_XGENE) += apm-mustang.dtb
 
-dtb-y += thunder-88xx.dtb
-
 targets += dtbs
 targets += $(dtb-y)