soc: qcom: Add Sleep stats driver

Let's add a driver to read the stats from remote processor and
export to debugfs.

The driver creates "qcom_sleep_stats" directory in debugfs and
adds files for various low power mode available. Below is sample
output with command

cat /sys/kernel/debug/qcom_sleep_stats/ddr
count = 0
Last Entered At = 0
Last Exited At = 0
Accumulated Duration = 0

Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>
Signed-off-by: Lina Iyer <ilina@codeaurora.org>
[mkshah: add subsystem sleep stats, create one file for each stat]
Signed-off-by: Maulik Shah <mkshah@codeaurora.org>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/1634107104-22197-3-git-send-email-mkshah@codeaurora.org