arm: mm: add functions to temporarily allow write to kernel text

STRICT_MEMORY_RWX write-protects the kernel text section. This
is a problem for tools such as kprobes which need write access
to kernel text space.

This patch introduces a function to temporarily make part of the
kernel text space writeable and another to restore the original state.
They can be called by code which is intentionally writing to
this space, while still leaving the kernel protected from
unintentional writes at other times.

Change-Id: I879009c41771198852952e5e7c3b4d1368f12d5f
Signed-off-by: Neil Leeder <nleeder@codeaurora.org>