net: macb: move pdata to private header

struct macb_platform_data is only used by macb_pci to register the platform
device, move its definition to cadence/macb.h and remove platform_data/macb.h

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>