arm64: Clarify when cpu_enable() is called

Strengthen the wording in the documentation for cpu_enable() to make it
more obvious to readers not already familiar with the code when the core
will call this callback and that this is intentional.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
[will: minor tweak to emphasis in the comment]
Signed-off-by: Will Deacon <will@kernel.org>