arm64: Document missing bitfields in cpu-feature-registers.rst

arm64: Document missing bitfields in cpu-feature-registers.rst

We have been rather lax in updating the list of visible bitfields in the
ID registers in cpu-feature-registers.rst, it is currently missing several
of the registers and quite a few bitfields in existing registers. Bring it
into sync with current -next.

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