drivers: soc: qcom: Add support to display RPMH master stats

Individual subsystem maintains stats on system sleep like sleep
counts, sleep last entered at, sleep last exited at, and accumulated
sleep duration.

Provide support to display it through sysfs.

Below command will display stats for each master available.
	cat /sys/power/rpmh_stats/master_stats

Sample output on SDM670.

MPSS
        Version:0x1
        Sleep Count:0x0
        Sleep Last Entered At:0x0
        Sleep Last Exited At:0x0
        Sleep Accumulated Duration:0x0

CDSP
        Version:0x1
        Sleep Count:0x2e
        Sleep Last Entered At:0x4aee0370
        Sleep Last Exited At:0x4aed513b
        Sleep Accumulated Duration:0x251c7019

Change-Id: I5224feed252e105c045ee028e4f56cbd244a101b
Signed-off-by: Maulik Shah <mkshah@codeaurora.org>