realtek: dsa: consolidate switch_irq()

realtek: dsa: consolidate switch_irq()

The dsa irq handler works always in the same way for all SoCs.

- Read register ISR_PORT_LINK_STS_CHG to determine the ports that
  triggered the irq.
- Write the read value back to the register to confirm the irq
- Read link status via MAC_LINK_STS
- Trigger dsa_port_phylink_mac_change() for each changed port

Currently each SoC has its own implementation. Drop that in
favour of a generic implementation that makes use of the existing
bit register read/write helpers.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/22273
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>