xen/arm: smmuv3: Import the SMMUv3 driver from Linux

xen/arm: smmuv3: Import the SMMUv3 driver from Linux

Based on tag Linux 5.8.18 commit ab435ce49bd1d02e33dfec24f76955dc1196970b

Directory structure change for the SMMUv3 driver starting from
Linux 5.9, to revert the patches smoothly using the "git revert" command
we decided to choose Linux 5.8.18.

Only difference between latest stable Linux 5.9.12 and Linux 5.8.18
SMMUv3 driver is the use of the "fallthrough" keyword. This patch will
be merged once "fallthrough" keyword implementation is available in XEN.

It's a copy of the Linux SMMUv3 driver. Xen specific code has not
been added yet and code has not been compiled.

Signed-off-by: Rahul Singh <rahul.singh@arm.com>
Acked-by: Julien Grall <jgrall@amazon.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>