stmmac: add a generic dwmac driver

Create a new driver around the generic device tree match strings
in the stmmac platform code. This driver is intended to be used
by all platforms that doesn't require any platform specific code
to function or is using platform data.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>