powerpc/82xx: rename and update mgcoge board support

The mgcoge board from keymile is now base for some other
similar boards. Therefore the board specific name mgcoge
was renamed to a generic name km82xx. Additionally some
enhancements were made:
	- rework partition table in dts file
	- add cpm2_pio_c gpio controller in dts file
	- update defconfig
	- add pin description for SCC1
	- add pin description and configuration for USB

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Acked-by: Heiko Schocher <hs@denx.de>
CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: Heiko Schocher <hs@denx.de>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>