msm-pm: Add support to manage Apps PLL outside linux

On some targets, every cpu is provided a clock controller phandle and
clock id. The pm driver uses this info to get a clock reference and
then use clock enable/disable to increase or decrease the reference
count when the going in and out of power collapse. When clock disable
is called and the reference count goes down to 0, the clock driver can
then turn off the pll to save power. On some targets the pll turning
off at power collapse introduces stability issues, due to glitches in
the clock mux and/or clock div hardware. On such targets the pll can
be managed outside of the linux power and clock drivers.

Add support to allow cpu clock management outside of the power, clock
driver, by ignoring failed attempts to acquire clock references at
probe. During probe each cpu can be expected to have a clock reference.
If the clock lookup fails, ignore it, instead of bailing out and failing
probe.

Change-Id: I22abc71268acda279dd1a454e08526b24d4c6879
Signed-off-by: Karthik Parsha <kparsha@codeaurora.org>