arm/km: update mgcoge3un board support

We change default settings for egiga on mgcoge3un.
The reason we need this is that we have the gig port on mgcoge3un
connected using a back-to-back pair of PHYs. There are no magnetics and
because of that the port has to be run with a fixd configuration and
auto-negotiation must be disabled. In the default mode the egiga driver
uses autoneg to determine port speed - which defaults to 1G (we need
100M full duplex).

Add wait for the GPIO line connected to mgcoge3ne before
starting mgcoge3un. A board specific ethernet present function
was added, because on this board ethernet is always present.
The BOCO FPGA access was enhanced and changed to use register
definitions.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Acked-by: Heiko Schocher <hs@denx.de>
cc: Wolfgang Denk <wd@denx.de>
cc: Detlev Zundel <dzu@denx.de>
cc: Prafulla Wadaskar <prafulla@marvell.com>