keymile/powerpc: move to the architecture-generic board system

This converts all the Keymile powerpc boards to the generic board
initialization.

This includes the 3 Keymile powerpc subsystems: km82xx, km83xx,
and kmp204x.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Reviewed-by: York Sun <yorksun@freescale.com>