DM9000: Add platform data to specify external phy

Patch from: Laurent Pinchart <laurentp@cse-semaphore.com>

This patch adds a flag to the DM9000 platform data which, when set,
configures the device to use an external PHY.

Signed-off-by: Laurent Pinchart <laurentp@cse-semaphore.com>
Signed-off-by: Ben Dooks <ben-linuy@fluff.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>