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

Documentation/ABI/testing/sysfs-devices-system-cpu

@@ -662,6 +662,7 @@ Description:	Preferred MTE tag checking mode
 
 		================  ==============================================
 		"sync"	  	  Prefer synchronous mode
+		"asymm"	  	  Prefer asymmetric mode
 		"async"	  	  Prefer asynchronous mode
 		================  ==============================================