net: marvell: mvpp2: add mvpp2_is_xlg() helper

Add a mvpp2_is_xlg() helper to identify whether the interface mode
should be using the XLGMAC rather than the GMAC.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>