powerpc/mpc85xx: fix TLB alignment

In the RAMBOOT/SPL case we were creating a TLB entry starting at
CONFIG_SYS_MONITOR_BASE, and just hoping that the base was properly
aligned for the TLB entry size.  This turned out to not be the case
with NAND SPL because the main U-Boot starts at an offset into the image
in order to skip the SPL itself.

Fix the TLB entry to always start at a proper alignment.  We still assume that
CONFIG_SYS_MONITOR_BASE doesn't start immediately before a large-page boundary
thus requiring multiple TLB entries.

Signed-off-by: Scott Wood <scottwood@frescale.com>
Cc: Andy Fleming <afleming@freescale.com>