xen/arm: smmuv3: Use fallthrough pseudo-keyword

xen/arm: smmuv3: Use fallthrough pseudo-keyword

Backport commit df561f6688fef775baa341a0f5d960becd248b11
"treewide: Use fallthrough pseudo-keyword" from Linux kernel.

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

Signed-off-by: Rahul Singh <rahul.singh@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>