ARM: dts: msm: Allow L2 to be in GDHS during Standalone PC for msm8974

Add a new low power level that puts L2 in GDHS mode during standalone
PC. This low power mode will give additional savings by putting L2 into
GDHS mode while the core does power collapse.

This can only be done in single core cases, the pm driver code takes
care of ensuring that L2 can be put into GDHS only when one core is online.

Change-Id: I71d237283820974c4d509606a01b67f5791d2fb9
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>