powerpc/mpc85xx:Fix README to show correct flash memory map

Due to increased size of u-boot, FMAN ucode start address has been shifted
by 256KB causing a overlap with rootfs start address.

Update rootfs start address to reflect correct memory map.

Also fix minor typo in README

Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>