regulator: core: Allow drivers to pass in a regmap

Since many regulators use regmap for register I/O and since there's quite
a few very common patterns in the code allow drivers to pass in a regmap
to the core for use in generic code.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@ti.com>