net: ti: am65-cpsw-nuss: Enforce pinctrl state on the MDIO child node

The binding represents the MDIO controller as a child device tree
node of the MAC device tree node.

The U-Boot driver mostly ignores that child device tree node and just
hardcodes the resources it uses to support both the MAC and MDIO in a
single driver.

However, some resources like pinctrl muxing states are thus ignored.
This has been a problem with some device trees that will put some
pinctrl states on the MDIO device tree node, like the SK-AM62 Device
Tree does.

Let's rework the driver a bit to create a dummy MDIO driver that we will
then get during our initialization to force the core to select the right
muxing.

Signed-off-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Acked-by: Roger Quadros <rogerq@kernel.org>
Acked-by: Nishanth Menon <nm@ti.com>