arm64: mmu: add ability to write kernel word

When the kernel text space is write-protected, provide
an API to allow debug tools to write a word to
kernel space. Temporarily remove the write-protection
for the protected area, write the word, and restore
the original protection.

CRs-Fixed: 705651
Change-Id: Idca4201214cca2a5b44742c9b9ef191352bfb6bb
Signed-off-by: Neil Leeder <nleeder@codeaurora.org>