Allow child classes of simplebus to call attach directly

Reduce code duplication when a bus is subclassed from simplebus by allowing
them to call simplebus_attach directly. This is useful when the child bus
will just implement the same calls.

As not all children will expect to have a ranges property, e.g. the
Raspberry Pi firmware, allow this property to be missing.

Reviewed by: manu
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D25925