85xx: bugfix for reading maximum TLB size on mpc85xx

The MAXSIZE field in the TLB1CFG register is 4 bits, not 8 bits.
This made setup_ddr_tlbs() try to set up a TLB larger than the e500 maximum
(256 MB)
which made u-boot hang in board_init_f() when trying to create a new stack
in RAM.
I have an mpc8540 with one 1GB dimm.

Signed-off-by: Fredrik Arnerup <fredrik.arnerup@edgeware.tv>
Signed-off-by: Andy Fleming <afleming@freescale.com>
Acked-by: Kumar Gala <galak@kernel.crashing.org>