spider_net: fix compile issue introduced by driver move

Both Spider net driver and Sun GEM driver use the sungem_phy.o object.
This fix creates a Kconfig object for sungem_phy (like MDIO) so that
both drivers require the SUNGEM_PHY object.

This has been compile tested for the Sun GEM driver.

Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>