t2080rdb/rcw: update ddr frequency from 1600MT/s to 1867MT/s

T2080RDB RevC uses new SODIMM 1867MT/s instead of previous 1600MT/s.
So update RCW to support new DDR frequency 1867MT/s by default.
Reserve the old 1600MT/s in comment for users in needed.

Signed-off-by: Shengzhou Liu <Shengzhou.Liu@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>