net: dsa: b53: Make SRAB driver manage port interrupts

Update the SRAB driver to manage per-port interrupts. Since we cannot
sleep during b53_io_ops, schedule a workqueue whenever we get a port
specific interrupt. We will later make use of this to call back into
PHYLINK when there is e.g: a link state change.

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