regulator: core: add regulator_list_corner_voltage function

Introduce a regulator_list_corner_voltage function for consumers
that need to know the maximum absolute voltage that may be
physically configured when a regulator is operating at a given
voltage corner.

Change-Id: Ide7a0b502796774fcad01f5cb5679f0938c72f96
Signed-off-by: David Collins <collinsd@codeaurora.org>