regulator: 88pm800: Depend on MFD_88PM800 rather than MFD_88PM800=y

This driver can be built as a module, make it depend on MFD_88PM800.
Otherwise, we are not able to select this driver when MFD_88PM800=m.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>