ARM: DRA7: EMIF: Add 4GB DDR settings

The REVH and later versions of DRA7-evm uses MICRON MT41K512M16HA-125 memory
chips which is of size 4GB(2GB on EMIF1 and 2GB on EMIF2). Add support for the
same.

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