Add the core of the code for dwge(4), a driver for the Synopsis Designware

Add the core of the code for dwge(4), a driver for the Synopsis Designware
GMAC core which is used on the Allwinner A20 SoCs and later SoCs in the
Allwinner family.  Since this core is very likely to show up the base of
other devices, it gets to live here in dev/ic.

Ported from NetBSD (where it is named awge(4)) by patrick@.  Some further
fixes by me.

ok deraadt@