Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add driver for DM&P Vortex86 RDC R6040 Fast Ethernet.
The controller is commonly found on DM&P Vortex86 x86 SoC. The
driver supports all hardware features except flow control. The
flow control was intentionally disabled due to silicon bug.

DM&P Electronics, Inc. provided all necessary information including
sample board to write driver and answered many questions I had.
Many thanks for their support of FreeBSD.

H/W donated by: DM&P Electronics, Inc.