Move support for Synopsys Designware APB UART out of ns8250 and into a
separate driver. Add support for activating clock and hwreset resources
for these devices when the EXT_RESOURCES option is present.

Reviewed by: andrew, mmel, Emmanuel Vadot <manu@bidouilliste.com>
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D5749