This patch programs the CPUECTLR_EL1 and L2ECTLR_EL1 registers,
so that the core waits for 512 generic timer CNTVALUEB ticks before
entering retention state, after executing a WFI instruction.

This functionality is configurable and can be enabled for platforms
by setting the newly defined 'ENABLE_L2_DYNAMIC_RETENTION' and
'ENABLE_CPU_DYNAMIC_RETENTION' flag.

Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>