msm: SSR: Add functionality to load restart groups from device tree

Subsystems on certain devices might need other subsystems to
restart when they undergo a subsystem restart.

The support to restart multiple subsystems together exists
currently. Use this feature to facilitate the restart groups
being initialized during run-time by reading the dependancy list
from the corresponding device tree entry using phandles.

Add a property called "qti,restart-group" in the respective dt
file device entries. This property is to be filled with the
phandles corresponding to the subsystems that need to restart
together.

Also, make a note that if a restart group is included for one
subsystem, the corresponding group should be included in the
other affected subsystems as well. The order of the subsystems
in the group doesn't matter.

Eg. subsys_0 has a property:
qti,restart-group = <&subsys_0 &subsys_1>;
Then, subsys_1's ssr-restart-group property should be identical:
qti,restart-group = <&subsys_1 &subsys_0>;

Change-Id: I22e3c46ab2b943b94227647d11863c4524a63005
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>