checks: Add bus checks for simple-bus buses

Add checks to identify simple-bus bus types and checks for child
devices. Simple-bus type is generally identified by "simple-bus"
compatible string. We also treat the root as a simple-bus, but only for
child nodes with reg property.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>