drivers: qcom: add RPM stats driver

RPM stats driver provides statistical information on the low power modes
entered by the SoC. The information is gathered form the number of times
the hardware blocks are configured to enter low power state and exported
by the always-on processor to other processors using shared memory.

This driver is based off the RPM stats driver available in 'commit
cc15c6703e90: (drivers: soc: qcom: rpm_stats: Switch to use
arch_counter_get_cntvct())' in the msm-4.4 branch.

Some changes made to the original include -
	- kmalloc/kzalloc -> kzalloc/devm_kzalloc
	- Whitespace issues addressed
	- Code re-org and cleanup
	- Add DT bindings documentation
	- Copyright updates
	- Use builtin platform driver instead of module
	- Kconfig documentation updates (MSM -> QTI)
	- rpm_stats.h is no longer needed.
	- Pruned old code and V1 format support

Change-Id: Ibceaa8f948e203c39e3df55b135c0a394f39ca5f
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>
Signed-off-by: Archana Sathyakumar <asathyak@codeaurora.org>
Signed-off-by: Lina Iyer <ilina@codeaurora.org>