drivers: regulators: depend on MFD_MAX8998

The MAX8998-pmic driver requires a core driver for all io operations. Like
other regulator driver make it depend of the required MFD io driver.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>