net: mvpp2: check first level interrupt status registers

Check the first level interrupt status registers to determine how to
further process the port interrupt. We will need this to know whether
to invoke the link status processing and/or the PTP processing for
both XLG and GMAC.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>