mfd: sun6i-prcm: Allow to compile with COMPILE_TEST

Since this driver only has a dependency on ARCH_SUNXI just because it
doesn't make any sense to run it on something else, we can definitely
enable it through COMPILE_TEST as well to get some build coverage.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>