net: dsa: mt7530: add interrupt support

Add support for MT7530 interrupt controller to handle internal PHYs.
In order to assign an IRQ number to each PHY, the registration of MDIO bus
is also done in this driver.

Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>