arm64/cpufeature: Unexport set_cpu_feature()

arm64/cpufeature: Unexport set_cpu_feature()

We currently export set_cpu_feature() to modules but there are no in tree
users that can be built as modules and it is hard to see cases where it
would make sense for there to be any such users. Remove the export to avoid
anyone else having to worry about why it is there and ensure that any users
that do get added get a bit more visiblity.

Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20220615191504.626604-1-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>