arm64/sme: Include ID_AA64PFR1_EL1.SME in cpu-feature-registers.rst

We expose ID_AA64PFR1_EL1.SME to userspace but do not document this in
cpu-feature-registers.rst. Add it.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230914-arm64-pfr1-sme-doc-v1-1-b6c497d10d77@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>