arm: mvebu: AC5/AC5X: use fixed page table size

Since commit 6cdf6b7a ("arm64: Use FEAT_HAFDBS to track dirty pages
when available") the default get_page_table_size() sets some flags for
more efficient handling of dirty page table entries. This causes
problems on the AC5/AC5X SoC (specifically a lockup when calling
__asm_switch_ttbr() via mmu_set_region_dcache_behaviour()).

The reason for the lockup isn't at all clear but it can be avoided if we
provide our own get_page_table_size() which stops gd->arch.has_hafdbs
from being set to true effectively returning the AC5/AC5X to the older
behaviour. This also opts for a simple fixed page table size rather than
trying to duplicate the more complicated logic to optimise the table
size.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>