drivers: soc: Allow building the sunxi driver without ARCH_SUNXI

This makes it possible to build the sunxi SRAM driver without building
for the sunxi architecture. This allows selecting the driver when
building the kernel in testing environments.

In particular, this is necessary for testing of the Cedrus driver, that
selects the sunxi SRAM driver.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>