drivers: qcom: Add new subsystem stats driver

This change adds subsystem sleep stats driver.
When userspace issues ioctl to read stats data, the driver
reads from smem and same passes to userspace.

Change-Id: Ia8ddb9096eb5f387922e8a7c0ad48ed71b736a85
Signed-off-by: Tushar Nimkar <tnimkar@codeaurora.org>