powerpc/t4240qds: fix offset of serdes when checking reference clock

powerpc/t4240qds: fix offset of serdes when checking reference clock

T4240 has 4 serdes, each serdes has 4k memory space, two PLLs.
We use PLL1CR0 to check the serdes reference clock.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>