arm: fix Kconfig symbol dependency on arm features

arm: fix Kconfig symbol dependency on arm features

The commit 3c2a14ea81c7 is introducing some unsupported arm features
that by default are disabled and are used for the cpufeature.c code.

As they are disabled by default, a typo in the Kconfig symbol they
depend on has landed in the codebase unnoticed, instead of depending
on ARM64 which does not exist, fix the code to depend on ARM_64 that
is the intended symbol.

Fixes: 3c2a14ea81c7 ("arm: Define kconfig symbols used by arm64 cpufeatures")
Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>