arm64: cpufeature: Group indexed system register definitions by name

Some system registers contain an index in the name (e.g. ID_MMFR<n>_EL1)
and, while this index often follows the register encoding, newer additions
to the architecture are necessarily tacked on the end. Sorting these
registers by encoding therefore becomes a bit of a mess.

Group the indexed system register definitions by name so that it's easier to
read and will hopefully reduce the chance of us accidentally introducing
duplicate definitions in the future.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>