arm: dts: msm8974: Enable L2 power collapse during sleep

The L2 power collapse was removed to address DDR corruption associated
with voltage droop on the MX rail. Power collapsing the MX rail doesn't
cause the Voltage to droop on the MX rail

CRs-fixed: 497712
Change-Id: Ie44b1f7cb0c36453c7cdcd479be79fde5d3ecbee
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>