ARM: EXYNOS5: Fix kernel dump in AFTR idle mode

The kernel crashes while resuming from AFTR idle mode. It happens
because L2 cache was not going into retention state.

This patch configures the USE_RETENTION bit of ARM_L2_OPTION register
so that it does not depend on MANUAL_L2RSTDISABLE_CONTROL of
ARM_COMMON_OPTION register for L2RSTDISABLE signal.

Signed-off-by: Inderpal Singh <inderpal.singh@linaro.org>
Tested-by: Chander Kashyap <chander.kashyap@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>