msm: rpm-regulator-smd: Add support for voltage corner resource parameter

Add support in the rpm-regulator-smd driver for the "corn"
voltage corner parameter.  It can be used with some regulators in
order to specify voltage performance corners instead of discrete
voltages.

Initial support is provided for MSM8974 SMPS 2 (VDD_Dig).

Change-Id: Icac67a1e5594d0aa54f3f6bd8aa8ac1edacc36d9
Signed-off-by: David Collins <collinsd@codeaurora.org>