MIPS: ath79: ar9331: add pause property for the MAC <> switch link

Both, MAC and switch support flow control, so add pause property for the
MAC <> switch link.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>