net: dsa: microchip: add ETS scheduler support for KSZ88x3 switches

net: dsa: microchip: add ETS scheduler support for KSZ88x3 switches

Implement Enhanced Transmission Selection scheduler (ETS) support for
KSZ88x3 devices, which support two fixed egress scheduling modes:
Strict Priority and Weighted Fair Queuing (WFQ).

Since the switch does not allow remapping priorities to queues or
adjusting weights, this implementation only supports enabling
strict priority mode. If strict mode is not explicitly requested,
the switch falls back to its default WFQ mode.

This patch introduces KSZ88x3-specific handlers for ETS add and
delete operations and uses TXQ Split Control registers to toggle
the WFQ enable bit per queue. Corresponding macros are also added
for register access.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://patch.msgid.link/20250410124249.2728568-1-o.rempel@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>