arm64: mmu: ensure tlb entry is flushed when making word writeable

A recent change to the tlb flush routine means it will not
flush ranges in the same page. Make the range one page long
so that the flush happens.

Change-Id: Ie7aa2065dbb7228483bb6332b3664ba18cc4d2c1
Signed-off-by: Neil Leeder <nleeder@codeaurora.org>