powerpc/mpc85xx: Reserve default boot page

The boot page in memory is already reserved so OS won't overwrite.
As long as the boot page translation is active, the default boot page
also needs to be reserved in case the memory is 4GB or more.

Signed-off-by: York Sun <yorksun@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>