mpc85xx/t2081: enable parsing DDR ratio for T2081 rev1.1

T2081 rev 1.1 changes MEM_PLL_RAT in RCW which requires new parsing
for PLL ratio.

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