Docs: dt: document ARM SMMUv3 generic binding usage

We're about to ratify our use of the generic binding, so document it.

CC: Rob Herring <robh+dt@kernel.org>
CC: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>