msm: pil-riva: Enable CXO clock to access PMU

The PMU registers are clocked from CXO out of power on reset.
Turn on the CXO clock so that these registers are accessible.
While we're here switch to the clk API instead of the XO buffer
interface.

Change-Id: Ia7e1be18531f8983368a037a9037c62c4d834403
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>