arm64/sysreg: Standardise naming for SSBS feature enumeration

arm64/sysreg: Standardise naming for SSBS feature enumeration

In preparation for conversion to automatic generation refresh the names
given to the items in the SSBS feature enumeration to reflect our standard
pattern for naming, corresponding to the architecture feature names they
reflect. No functional change.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Kristina Martsenko <kristina.martsenko@arm.com>
Link: https://lore.kernel.org/r/20220905225425.1871461-16-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>