Add compatibility string for dw-apb-uart in ns8250 driver

This compatibility string is used in .dts file of Armada38x
and isrequired for driver attachment.

Reviewed by: andrew, ian, imp
Obtained from: Semihalf
Sponsored by: Stormshield
Submitted by: Michal Stanek <mst@semihalf.com>
Differential revision: https://reviews.freebsd.org/D4216