msm: avs: Add API to restore the AVSCSR register

During power collapse, the AVSCSR register is always enabled and never
restored. This could result in AVS getting enabled on resume, when the
core is running at a frequency that doesn't support AVS.

Add new API, avs_enable, which restores the AVSCSR register based on the
saved AVSCSR register. Update the avs_disable API, to return the
AVSCSR register value prior to disabling the AVS.

(cherry picked from commit ad138da17fe08d46412d56b84309c5d17f7d5931)

Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>

Change-Id: I2bd2e1d6aea058e0f44c474da6c3121654f1236e
Signed-off-by: Neha Pandey <nehap@codeaurora.org>