arm64: Document boot requirements for PIE

Features PIE and TCR2 introduce new registers, update the trap requirements
for these features.

Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20230606145859.697944-19-joey.gouly@arm.com


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