mfd: da9063: Allow modular build

Allow support for the DA9063 PMIC to be modular, cfr. DA9062, which
allows to decrease size of multi-platform kernels (e.g.
multi_v7_defconfig).

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Steve Twiss <stwiss.opensource@diasemi.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>