The macb/gem core is used by the Zynq SoC.  In preparation of sharing
the macb driver between at91 and Zynq, rename the platform data to
'struct macb_platform_data', and move the definition to a common
location.

Signed-off-by: Josh Cartwright <joshc@eso.teric.us>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>