arm64: docs: cpu-feature-registers: Document ID_AA64PFR1_EL1

Commit d71be2b6 ("arm64: cpufeature: Detect SSBS and advertise
to userspace") exposes ID_AA64PFR1_EL1 to userspace, but didn't
update the documentation to match.

Add it.

Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>