eth: dw: Add driver for Synopsys DesignWare Ethernet MAC

This patch adds a driver for a Synopsys DesignWare Ethernet MAC.  The
driver uses interrupts to handle received frames, but it uses a
spinloop when transmitting to wait for the transmit descriptor to
become available.  Transmission is coordinated by a fiber, so this
should not result in the system execution being blocked.  Only a
single descriptor is allocated for each of the transmit and receive
directions to save memory and simplify the code.  Another
simplification is that none of the offload capabilities of the
Ethernet device are used.  The driver currently only supports a single
instance of the Ethernet MAC, which is consistent with the limitation
in the network stack that only a single network device is supported.

Change-Id: I013b3d439a76e8ff91a775516f7035841b040870
Signed-off-by: Michael LeMay <michael.lemay@intel.com>