FROMLIST: arm64: mte: introduce a per-CPU tag checking mode preference

Add a per-CPU sysfs node, mte_tcf_preferred, that allows the preferred
tag checking mode to be configured. The current possible values are
async and sync.

Link: https://linux-review.googlesource.com/id/I7493dcd533a2785a1437b16c3f6b50919f840854


Signed-off-by: Peter Collingbourne <pcc@google.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/linux-arm-kernel/20210713234801.3858018-5-pcc@google.com/
Bug: 189966263
Change-Id: I7493dcd533a2785a1437b16c3f6b50919f840854