armv7m: FAULTMASK should be 0 on reset

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 <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Message-id: 1485285380[email protected]
[PMM: rewrote commit message, moved comments]
Signed-off-by: Peter Maydell <[email protected]>