drivers: qcom: system_pm: add system PM driver for QTI SoC

Handle QTI specific functionality when entering system low power modes
for the SoC. When the application processor subsystem, decides it has
nothing else to do, it could enter an idle state initiated either by
cpuidle or suspend framework. In both these cases, Linux is ready to
vote for the SoC to enter deeper low power modes that could turn off or
lower requirements on resources that are no longer needed by the
subsystem. This facilitates power saving by the SoC by reducing
bandwidth requirements on DDR, turning off backbone power rails, XO
buffers etc.

The driver exports API that are called by the cpuidle drivers that when
it has been decided that the system can safely enter deep idle states
for enough duration that it makes powering down SoC components
worthwhile.

This is a snapshot as of commit b4a69c05b01172d4be245db80f4b9.

Change-Id: I3b8ba0077b4d9abaa00ab8843104fa02b816afdd
Signed-off-by: Lina Iyer <ilina@codeaurora.org>