arm64: booting: Document our requirements for fine grained traps with SME

With SME we require that fine grained traps on access to TPIDR2_EL0 and
SMPRI_EL1 are disabled but did not document that fact. Add the relevant
register bits.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221101112716.52035-2-broonie@kernel.org