net: mvpp2: add port support helpers

The mvpp2 code has tests scattered amongst the code to determine
whether the port supports the XLG, and whether the port supports
RGMII mode.

Rather than having these tests scattered, provide a couple of helper
functions, so that future additions can ensure that they get these
tests correct.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>