docs: sysfs-devices-system-cpu: document "asymm" value for mte_tcf_preferred

It was added in commit 766121ba ("arm64/mte: Add userspace interface
for enabling asymmetric mode").

Signed-off-by: Evgenii Stepanov <eugenis@google.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220309215943.87831-1-eugenis@google.com


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