ARM: cpaccess: write-enables kernel space for write

Calls the mmu function to ensure that kernel space is
write-enabled before writing the updated CP15
instruction. Removes the unnecessary write of the
constant bx lr instruction.

Change-Id: Idfe44bb89e793b009b17565e7486f3a6e8cdf582
Signed-off-by: Neil Leeder <nleeder@codeaurora.org>
(cherry picked from commit 7673a8753ade677b02fbc013f95958d2ee815ba0)