ARM: PMU: fix runtime PM enable

Commit 7be2958e (ARM: PMU: Add runtime PM Support) updated the ARM PMU code to
use runtime PM which was prototyped and validated on the OMAP devices. In this
commit, there is no call pm_runtime_enable() and for OMAP devices
pm_runtime_enable() is currently being called from the OMAP PMU code when the
PMU device is created. However, there are two problems with this:

1. For any other ARM device wishing to use runtime PM for PMU they will need
   to call pm_runtime_enable() for runtime PM to work.
2. When booting with device-tree and using device-tree to create the PMU
   device, pm_runtime_enable() needs to be called from within the ARM PERF
   driver as we are no longer calling any device specific code to create the
   device. Hence, PMU does not work on OMAP devices that use the runtime PM
   callbacks when using device-tree to create the PMU device.

Therefore,  call pm_runtime_enable() directly from the ARM PMU driver when
registering the device. For platforms that do not use runtime PM,
pm_runtime_enable() does nothing and for platforms that do use runtime PM but
may not require it specifically for PMU, this will just add a little overhead
when initialising and uninitialising the PMU device.

Tested with PERF on OMAP2420, OMAP3430 and OMAP4460.

Acked-by: Kevin Hilman <khilman@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>