85xx-fdt: Fixed l2-ctlr's compatible prop for QorIQ

The code assumed names where just numbers and always prefixed 'mpc'.
However newer QorIQ don't follow the mpc naming scheme.

Signed-off-by: Vivek Mahajan <vivek.mahajan@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>