net: dsa: mv88e6xxx: introduce .serdes_irq_enable

Introduce a new .serdes_irq_enable operation to prepare the abstraction
of IRQ enabling from the SERDES IRQ setup code.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>