fix: remove "experimental" tag for stable features

there are features which are marked as experimental even though they
are stable and used for quite some time.
Following features are no longer marked as experimental
  - SPMD
  - MEASURED_BOOT
  - FCONF and associated build flags
  - DECRYPTION_SUPPORT and associated build flags
  - ENABLE_PAUTH
  - ENABLE_BTI
  - USE_SPINLOCK_CAS
  - GICv3 Multichip support

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: I4bb653d9c413c66095ec31f0b8aefeb13ea04ee9
7 files changed