[ARM] Orion: use mv643xx_eth driver mbus window handling

Make the Orion 5x platform code use the mbus window handling code
that's in the mv643xx_eth driver, instead of programming the GigE
block's mbus window registers by hand.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Reviewed-by: Tzachi Perelstein <tzachi@marvell.com>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Nicolas Pitre <nico@marvell.com>