arm64: Add RV and RN fields for ESR_ELx_WFx_ISS

The ISS field exposed by ESR_ELx contain two additional subfields
with FEAT_WFxT:

- RN, the register number containing the timeout
- RV, indicating if the register number is valid

Describe these two fields according to the arch spec.

No functional change.

Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20220419182755.601427-3-maz@kernel.org