arm64/esr: Document ISS for ZT0 being disabled

SME2 defines a new ISS code for use when trapping acesses to ZT0, add a
definition for it.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221208-arm64-sme2-v4-5-f2fa0aef982f@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>