target/arm: Remove incorrect comment about MPU_CTRL

target/arm: Remove incorrect comment about MPU_CTRL

Remove the comment that claims that some MPU_CTRL bits are stored
in sctlr_el[1]. This has never been true since MPU_CTRL was added
in commit 29c483a50607 -- the comment is a leftover from
Michael Davidsaver's original implementation, which I modified
not to use sctlr_el[1]; I forgot to delete the comment then.

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Edgar E. Iglesias <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Message-id: 1501692241[email protected]