regulator: vexpress: Get rid of struct vexpress_regulator

The *regdev and *regmap can be replaced by local variables in probe().
Only desc of struct vexpress_regulator is really need, so just use
struct regulator_desc directly and remove struct vexpress_regulator.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>