arm: cache: always flush cache line size for page table

arm: cache: always flush cache line size for page table

The page table is maintained by the CPU, hence it is safe to always
align cache flush to a whole cache line size. This allows to use
mmu_page_table_flush for a single page table, e.g. when configure
only small regions through mmu_set_region_dcache_behaviour.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Tested-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>