regulator: rpm-smd-regulator: add support for enable via pin control

Add support for sending dynamic requests for the enable pin
control parameter 'pcen' via regulator_enable() and
regulator_disable() calls.  This is managed via a new device tree
property which specifies the 'pcen' value to send for both the
disabled and enabled logical states.

Change-Id: I80744f112298f6de3761ac1868a0589686f633a2
Signed-off-by: David Collins <collinsd@codeaurora.org>