kirkwood: define empty CONFIG_MVGBE_PORTS by default

Each board with defines it's own set of values. If we do not define
CONFIG_MVGBE_PORTS we will hit following error:

mvgbe.c: In function 'mvgbe_initialize':
mvgbe.c:700:34: error: 'CONFIG_MVGBE_PORTS' undeclared (first use in this function)
  u8 used_ports[MAX_MVGBE_DEVS] = CONFIG_MVGBE_PORTS;

This patch fixes above described problem.

Signed-off-by: Luka Perkov <luka@openwrt.org>
Acked-by: Prafulla Wadaskar <prafulla@marvell.com>