msm: spm-v2: Add support for AVS

Changing the voltage with the AVS feature enabled requires
certain care. First, temporarily disable AVS before writing any
registers which would change the voltage. Then kick the PMIC
state machine back to the idle state so that our write to the
VCTL register takes effect immediately. We don't need to check
the state machine for idle, instead we should just confirm that
the SPM has sent the voltage we wanted to send and then we can
program AVS to allow up to 4 12.5mV steps down from the voltage
programmed in VCTL. Once this is all done reenable AVS so that
the voltage can be decreased after the AVS_DELAY counts down.

Change-Id: I1c23fd0549e401152fd16de9032ceaf7c3e067c4
Signed-off-by: Praveen Chidambaram <pchidamb@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>