T4240: Address T4240/T4160 Rev2.0 DDR clock change

MEM_PLL_RAT on T4240/T4160 Rev2.0 uses a value which is half of Rev1.0.
It's 12 in Rev1.0, for Rev2.0 it uses 6.

Signed-off-by: Roy Zang <tie-fei.zang@freescale.com>
Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Acked-by: York Sun <yorksun@freescale.com>