mmu: arm64: fix ability to write to protected memory

Use the mem_text_address_writeable function if
CONFIG_KERNEL_TEXT_RDONLY is specified. Modify the
page table entry rather than the pmd, depending
on pmd type.

Change-Id: I04390a9b7376b299161842e87150802da2d4d728
Signed-off-by: Neil Leeder <nleeder@codeaurora.org>