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>