mfd: abx500-core: Allow driver to built if COMPILE_TEST is enabled

The driver only has runtime but no build time dependency with ARCH_U300 ||
ARCH_U8500 So it can be built for testing purposes if COMPILE_TEST option
is enabled.

This is useful to have more build coverage and make sure that the driver
is not affected by changes that could cause build regressions.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>