arm64: fix ID_AA64ISAR2_WFxT_IMPL value

Bits 3:0 of ID_AA64ISAR2_EL1 are reserved for FEAT_WFxT in ARMv8.7, with
bit 1 used to determine whether the feature is supported. The macro,
ID_AA64ISAR2_WFxT_IMPL can be used to check if this bit is set, however
it is currently incorrectly set to check bit 0. This has now been
corrected.

Reviewed by: andrew
Sponsored by: Arm Ltd
Signed-off-by: Harry Moulton <harry.moulton@arm.com>
Differential Revision: https://reviews.freebsd.org/D48577