regulator: mcp16502: Remove unneeded fields from struct mcp16502

At the context with rdev, we can use rdev->regmap instead of mcp->rmap.
The *rdev[NUM_REGULATORS] is not required because current code uses
devm_regulator_register() so we don't need to store *rdev for clean up
paths.

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