dt-bindings: net: document Microchip PIC64-HPSC/HX MDIO controller

dt-bindings: net: document Microchip PIC64-HPSC/HX MDIO controller

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 separate documentation.

The hardware supports C22 and C45.

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 to signal transaction completion
which is not strictly needed as the software can also poll a "TRIGGER"
bit for this.

Signed-off-by: Charles Perry <charles.perry@microchip.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20260408131821.1145334-2-charles.perry@microchip.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>