Some MAC PCS blocks are unable to provide interrupts when their status
changes. As we already have support in phylink for polling status, use
this to provide a hook for MACs to enable polling mode.

The patch idea was picked up from Russell King's suggestion on the macb
phylink patch thread here [0] but the implementation was changed.
Instead of introducing a new phylink_start_poll() function, which would
make the implementation cumbersome for common PHYLINK implementations
for multiple types of devices, like DSA, just add a boolean property to
the phylink_config structure, which is just as backwards-compatible.

https://lkml.org/lkml/2019/12/16/603Suggested-by: NRussell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: NVladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>