mfd: pm8xxx: Make elegible for COMPILE_TEST

This should be enabled so that we get full compile coverage
of the PM8xxx MFD core with the different subdrivers.

Tested on the build servers.

Suggested-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>