regulator: core: Try using the parent device for the default regmap

If the device doesn't have a regmap specified by the driver and we can't
find one on the device itself try its parent, providing a useful defualt
for many MFDs.

[Rewrite commit message -- broonie]

Signed-off-by: AnilKumar Ch <anilkumar@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>