msm: lpm: Add documentation for cluster lpm clock gating mode

Cluster level SPMs are able to support Clock Gating along with the other
low power modes. Add documentation to reflect this support.

Change-Id: Icfe2139104703f8f10192cfd9298c94c5ea8ceab
Signed-off-by: Karthik Parsha <kparsha@codeaurora.org>