bus: sunxi-rsb: Enable by default for ARM64

Allwinner's A64 SoC uses the "Reduced Serial Bus" to communicate with
its companion PMIC.

Since arm64 does not have separate defconfigs for each platform or
processor family, enable this driver by default for ARM64 as well.
Note that the Kconfig symbol already depends on ARCH_SUNXI.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
[wens@csie.org: Refined commit message]
Signed-off-by: Chen-Yu Tsai <wens@csie.org>