keymile boards: add CONFIG_PIGGY_MAC_ADRESS_OFFSET

Normaly the PIGGY_MAC_ADRESS can be read directly from the
IVM on keymile boards. On mgcoge3 it differs. Because there
are two piggy boards deployed the second MAC adress must be
calculated with the IVM mac adress and an offset. This patch
allows to set such a offset in the board config.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
cc: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Heiko Schocher <hs@denx.de>