Add feature register flag definitions to clarify which features
might be supported.

Consolidate the various ID_AA64PFR0_ELx flags for all ELs.

No functional change intended.
Signed-off-by: Fuad Tabba <tabba@google.com>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210817081134.2918285-10-tabba@google.com