arm64: sysreg: Describe ID_AA64DFR2_EL1 fields

Describe the new ID register in line with DDI0601 2024-09.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20241025182354.3364124-3-oliver.upton@linux.dev


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>