lpm-levels: Add support for APC off with L2RAM ON mode

To support APCS_PMIC_OFF_L2RAM_ON mode, add support for
device tree entry to specify if L2 cache need not be flushed.

Change-Id: Id67c97359f73ea364d10f81b1d2623f2dcfe820a
Signed-off-by: Srinivas Rao L <lsrao@codeaurora.org>