msm: clock-8974: Hold an active set vote for CXO in post_init

The CXO is the only crystal source for the MSM on the
8974. Thus as long as the apps processor is awake,
the CXO clock is expected to be on. To be safe, vote
for CXO in the active set in post_init, which is the
earliest point that this vote can be asserted.

Change-Id: Ib38e97d9c44fc60d0ce898a8d600519a327bb66a
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>