Make uart_bus_fdt a decendant of ofwbus

This will allow to attach UART drivers lying directly on the root node
instead of simple-bus compatible bus only.

Obtained from:   Semihalf
Sponsored by:    The FreeBSD Foundation


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274452 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f