bus/arm-cci: Streamline devicetree handling a bit

Rather than iterating over child nodes explicitly testing for
availability, we can just use the other helper which already subsumes
that check. Also, the availability check is already NULL-safe, so get
rid of a redundant check in cci_probe(), too.

Acked-by: Punit Agrawal <punit.agrawal@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>