arm64: add inline functions for FEAT_WFxT instructions

This adds inline functions for WFET/WFIT instructions introduced with
ARMv8.7 FEAT_WFxT. These instructions allow for regular WFE/WFI
instructions to be bound by a timeout. Once the value of CNTVCT_EL0
reaches, or exceeds, that of the timeout value, the WFE/WFI state will
be exited and the PE will continue.

These instructions are implemented as inline functions, with a write to
the corresponding system register acting as an alias for the two
instructions. Writing to these registers is only valid on ARMv8.7 and
above, therefore the use of these functions must be guarded by a feature
check.

Reviewed by: andrew
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D48579
Signed-off-by: Harry Moulton <harry.moulton@arm.com>