arm64/mte: Add a little bit of documentation for mte_update_sctlr_user()

The code isn't that obscure but it probably won't hurt to have a little
bit more documentation for anyone trying to find out where everything
actually takes effect.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Vincenzo Frascino <Vincenzo.Frascino@arm.com>
Tested-by: Branislav Rankov <branislav.rankov@arm.com>
Link: https://lore.kernel.org/r/20220216173224.2342152-3-broonie@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>