arm/km: rename mgcoge2un to mgcoge3un

arm/km: rename mgcoge2un to mgcoge3un

The mgcoge2un target was only an intermediate step to mgcoge3un.
For this reason the mgcoge2un support was moved to mgcoge3un,
because it isn't needed to support both targets.

We add the BootROM init file for the mgcoge3un memphis RAM.

We also move the suen3 and suen8 boards into the correct category
in the MAINTAINERS file.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@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>