regulator: cpr_regulator: Use devm_regulator_get_optional for vdd_apc

devm_regulator_get returns a dummy regulator handle if the required
regulator is not found. Instead use devm_regulator_get_optional to
parse through list of vdd_apc regulator names.

Change-Id: I081a6e2579cc3db9d9d7c779cc12ca06fa418fd7
Signed-off-by: Ravindranath Thiyagarajan <rthiyaga@codeaurora.org>