net: mdio: add a driver for PIC64-HPSC/HX MDIO controller

net: mdio: add a driver for PIC64-HPSC/HX MDIO controller

This adds an MDIO driver for PIC64-HPSC/HX. The hardware supports C22
and C45 but only C22 is implemented in this commit.

This MDIO hardware is based on a Microsemi design supported in Linux by
mdio-mscc-miim.c. However, The register interface is completely
different with pic64hpsc, hence the need for a separate driver.

The documentation recommends an input clock of 156.25MHz and a prescaler
of 39, which yields an MDIO clock of 1.95MHz.

The hardware supports an interrupt pin or a "TRIGGER" bit that can be
polled to signal transaction completion. This commit uses polling.

This was tested on Microchip HB1301 evalkit with a VSC8574 and a
VSC8541.

Signed-off-by: Charles Perry <charles.perry@microchip.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20260408131821.1145334-3-charles.perry@microchip.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>