arm64: Add wfet()/wfit() helpers

Just like we have helpers for WFI and WFE, add the WFxT versions.
Note that the encoding is that reported by objdump, as no currrent
toolchain knows about these instructions yet.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20220419182755.601427-10-maz@kernel.org