powerpc/mpc85xx: Avoid hardcoding in SPL linker script

SPL linker has fix location of bootpg and reset vector with respect to text base.
It is not necessary to have fixed locations.

Avoid such hardcoding.

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