ARM: OMAP4+: Cleanup emif specific files

Removing the duplicated code in ddr3 initialization.
Also creating structure for lpddr2 mode registers to
avoid unnessecary revision checks.

These change reduces code addition for future Socs.

Signed-off-by: R Sricharan <r.sricharan@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@ti.com>