arm64/mte: Document ABI for asymmetric mode

MTE3 adds a new mode which is synchronous for reads but asynchronous for
writes. Document the userspace ABI for this feature, we call the new
mode ASYMM and add a new prctl flag and mte_tcf_preferred value for it.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20220216173224.2342152-2-broonie@kernel.org


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