powerpc: keymile: Add a check for the PIGGY debug board Check the presence of the PIGGY on the keymile boards mgcoge, mgsuvd and kmeter1. If the PIGGY is not present, dont register this Ethernet device. Signed-off-by: Heiko Schocher <hs@denx.de> Acked-by: Ben Warren <biggerbadderben@gmail.com>