net: dsa: mv88e6xxx: add number of Global 2 IRQs

Similarly to g1_irqs, add a g2_irqs member to the info structure to
indicates the presence of the Global 2 Interrupt Source and Mask
registers.

At the same time, provide helpers and document the registers since they
differ a bit between 88E6352 and 88E6390 families.

This allows us to get rid of the MV88E6XXX_FLAG_G2_INT flag.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>