feat: add register definitions needed for SMCCC_ARCH_FEATURE_AVAILABILITY

Also slightly optimised some redundant feature functions

Co-developed-by: Charlie Bareham <charlie.bareham@arm.com>
Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I6dcc11060a2f3697a8aa41443e9cfc665b2b7c74
9 files changed