msm: spm: Add support to use SPM for clock gating mode

On targets that support auto clock gating, spm needs to be disabled
for the hardware to do auto clock gating. If auto clock gating is not
supported or not used, spm needs to be left enabled when programmed
for clock gating.

Add driver support to keep the spm enabled, and, if qchannel interface
is present then add driver support to enable spm legacy mode. This
would keep the spm enabled for clock gating. Use the dt flag
qcom,use-spm-clock-gating to determine if spm needs to be used for
clock gating.

Also, fix lpm-levels driver to support clock gating and auto clock gating
modes.

Change-Id: I4c796d7e063633e41910e780ee2a05e5205db0bd
Signed-off-by: Karthik Parsha <kparsha@codeaurora.org>