Enable PAN emulation using TTBR0_EL1 switching.

Reviewed-at: https://android-review.googlesource.com/#/c/325997/Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
[AmitP: cherry-picked this change from Android common kernel
        and updated the commit message]
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>