regulator: core: Use dev_get_regmap() to find the regmap

If no regmap is explicitly specified then use dev_get_regmap() to obtain
one. The driver must explicitly enable any actual usage of the regmap
so there's no concern with unwanted usage.

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