regulator: cpr3-regulator: introduce cpr3_regulator_ldo_apm_prepare()

Add cpr3_regulator_ldo_apm_prepare() to handle LDO regulator
configuration that must occur as a last step before APM switches
occur. Refactor the logic which ensures that the LDO maximum
headroom is not violated when the APM is switched to the
system-supply voltage source into this function.

Change-Id: I884dfc7ba4fb05cf084f8d8a63a86684473fd67a
Signed-off-by: Osvaldo Banuelos <osvaldob@codeaurora.org>