powerpc/p2041: fix I2C controller's offset

Without this patch, SPD access will fail which leads to DDR init fail.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Tested-by: Chris Packham <judge.packham@gmail.com>
Acked-by: York Sun <yorksun@freescale.com>