On the LS1021A-TSN board, the 2 Atheros AR8031 PHYs for eth0 and eth1
have interrupt lines connected to the shared IRQ2_B LS1021A pin.

Switching to interrupts offloads the PHY library from the task of
polling the MDIO status and AN registers (1, 4, 5) every second.

Unfortunately, the BCM5464R quad PHY connected to the switch does not
appear to have an interrupt line routed to the SoC.
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>