net: dsa: mv88e6xxx: Select IRQ_DOMAIN

Some architectures may not define IRQ_DOMAIN (like m32r), fixes
undefined references to IRQ_DOMAIN functions.

Fixes: dc30c35b ("net: dsa: mv88e6xxx: Implement interrupt support.")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>