msm: pm-8x60: Expose additional APIs through pm.h

The following two non-static APIs should be exposed in a header
file but are not. Fix this. When the pm-8x60 driver is not used,
use stubs in their place that always return success.

    int msm_pm_wait_cpu_shutdown(unsigned int cpu)
    bool msm_pm_verify_cpu_pc(unsigned int cpu)

msm_pm_set_rpm_wakeup_irq(unsigned int irq) is also specific to
the pm-8x60 driver, so stub it out for targets without pm-8x60,
too.

Change-Id: I257a458a7e435b5cbc3b058a9fb245ea8704ca1a
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>