arm64: Introduce Allwinner SoC config option

To prepare for supporting the Allwinner A64 SoC, introduce a config
option to allow compiling Allwinner (aka. sunxi) specific drivers
for ARM64.
This patch just defines the ARCH_SUNXI symbol to allow Allwinner
specific drivers to be selected during kernel configuration.
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

arch/arm64/Kconfig.platforms

 menu "Platform selection"
 
+config ARCH_SUNXI
+	bool "Allwinner sunxi 64-bit SoC Family"
+	help
+	  This enables support for Allwinner sunxi based SoCs like the A64.
+
 config ARCH_BCM_IPROC
 	bool "Broadcom iProc SoC Family"
 	help