arm64: Kconfig: Spell out "ARMv9.4" in menuconfig text

The menuconfig entries to configure various architectural features are
all formatted as "ARMvx.y architecture features" with the unusual
exception of 9.4, which omits the "ARM" prefix.

Add the "ARM" prefix to the menuconfig entry for the ARMv9.4
architectural features.

Signed-off-by: Will Deacon <will@kernel.org>