ARM: DRA7: DDR: Enable SR in Power Management Control

If EMIF is idle for certain amount of DDR cycles, EMIF will put the
DDR in self refresh mode to save power if EMIF_PWR_MGMT_CTRL register
is programmed. And also before entering suspend-resume ddr needs to
be put in self-refresh. Linux kernel does not program this register
before entering suspend and relies on u-boot setting.
So configuring it in u-boot.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Tested-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>