cmn600: Stop checking type of external nodes

Registers belonging to external nodes cannot be accessed if the power
domain that the node belongs to is not on. We therefore need to avoid
calling `get_node_type()` on external nodes, as doing so will fail.

Change-Id: Iac405c9f4c7095018cfec79185e6b84dece81e73
Signed-off-by: Chris Kay <chris.kay@arm.com>