powerpc/mpc85xx:Update MONITOR_LEN for 768KB u-boot size
U-boot binary size has been increased from 512KB to 768KB.
So update CONFIG_SYS_MONITOR_LEN to reflect the same.
Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>