powerpc/mpc85xx: Update B4 FMan MURAM size

FMan V3H has 2 different MURAM sizes:
    In B4860/4420 the MURAM size is 512KB.
    In T4240 and T2080 the MURAM size is 384KB.

The MURAM size in FMan V3H device tree is 384KB.
This patch updates the MURAM size for B4 to 512KB.

Signed-off-by: Igal Liberman <igal.liberman@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>