AVR32: macb - Search for PHY id

This patch adds support for searching through available PHY-addresses in
the macb-driver. This is needed for the ATEVK1100 evaluation board,
where the PHY-address will be initialized to either 1 or 7.

This patch adds a config option, CONFIG_MACB_SEARCH_PHY, which when
enabled tells the driver to search for the PHY address.

Signed-off-by: Gunnar Rangoy <gunnar@rangoy.com>
Signed-off-by: Paul Driveklepp <pauldriveklepp@gmail.com>
Signed-off-by: Olav Morken <olavmrk@gmail.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>