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>

drivers/net/dsa/mv88e6xxx/Kconfig

 config NET_DSA_MV88E6XXX
 	tristate "Marvell 88E6xxx Ethernet switch fabric support"
 	depends on NET_DSA
+	select IRQ_DOMAIN
 	select NET_DSA_TAG_EDSA
 	select NET_DSA_TAG_DSA
 	help