dt-bindings: net: mscc-miim: add lan966x compatible

The MDIO controller has support to release the internal PHYs from reset
by specifying a second memory resource. This is different between the
currently supported SparX-5 and the LAN966x. Add a new compatible to
distinguish between these two.

Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>