ARM: OMAP2+: PMU: Add runtime PM support

ARM: OMAP2+: PMU: Add runtime PM support

The original implementation of this patch was done by Ming Lei for PMU on OMAP4
[1]. Since then the PM runtime calls have been moved into the ARM PMU code and
this greatly simplifies the changes.

The another differnce since the original version, is that it is no longer
necessary to call pm_runtime_get/put during the PMU initialisation was we are no
longer accessing the hardware at this stage.

By adding runtime PM support, we can ensure that the appropriate power and clock
domains are kept on while PMU is being used.

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2011-November/074153.html

Cc: Ming Lei <ming.lei@canonical.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Benoit Cousson <b-cousson@ti.com>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: Kevin Hilman <khilman@ti.com>
Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>