arm64: enable generic idle loop

Enable generic idle loop for ARM64, so can support for hlt/nohlt
command line options to override default idle loop behavior.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>