arm/dt: msm8974: Set retention as default L2 low power mode

Due to a hardware bug on MSM8960, L2 retention was disabled when the
Krait cores execute clock gating/standalone power collapse. The MSM8974
chipsets dont have this bug and setting the default mode to retention
would result in additional power savings.

Change-Id: I2bc4367ed456ac5b7b6643b426a120c303693d71
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>