power: qcom: apm: add msm_apm_get_supply() to determine APM mode

Add a function which consumers can call to determine the current
APM supply selection. This is useful in scenarios where consumer
drivers need to determine what the current APM mode is.

Change-Id: I16b7e6814ef98f2119899451f8700998d8ee9c9f
Signed-off-by: Osvaldo Banuelos <osvaldob@codeaurora.org>