armv7m: FAULTMASK should be 0 on reset

For M profile CPUs, FAULTMASK should be 0 on reset, like PRIMASK.
QEMU stores FAULTMASK in the PSTATE F bit, so (as with PRIMASK in the
I bit) we have to clear these to undo the A profile default of 1.

Update the comment accordingly and move it so that it's closer to the
code it's referring to.

Signed-off-by: Michael Davidsaver <mdavidsaver@gmail.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1485285380-10565-10-git-send-email-peter.maydell@linaro.org
[PMM: rewrote commit message, moved comments]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>