arm64: imx8mp: Inhibit DTC warning on DH i.MX8MP DHCOM rev.100 DTO

Inhibit DTC warning in imx8mp-dhcom-pdk3-overlay-rev100.dts:
"
arch/arm/dts/imx8mp-dhcom-pdk3-overlay-rev100.dtbo: Warning (reg_format): /fragment@0/__overlay__:reg: property has invalid length (4 bytes) (#address-cells == 2, #size-cells == 1)
arch/arm/dts/imx8mp-dhcom-pdk3-overlay-rev100.dtbo: Warning (avoid_default_addr_size): /fragment@0/__overlay__: Relying on default #address-cells value
arch/arm/dts/imx8mp-dhcom-pdk3-overlay-rev100.dtbo: Warning (avoid_default_addr_size): /fragment@0/__overlay__: Relying on default #size-cells value
"

The DTO overwrites the 'reg' property of an ethernet PHY and is only
used on specific combination of old prototype SoM and old prototype
PDK3 carrier board, which had incorrectly placed pull resistor, which
made the PHY change its MDIO address in that specific combination and
which is already fixed on production hardware.

The DTO is implemented in this simple manner because if it contained a
full MDIO bus node reference to define #address-cells and #size-cells,
it would also require a full new copy of the PHY node, i.e.
ethernet-phy@5 { ... reg = <5>; ... }, to avoid DTC warnings about
mismatch between node unit and reg value. The node unit in SoM DT is
ethernet-phy@7 { ... }; .

This simpler approach avoids unnecessary duplication without adverse
side effects.

Reported-by: Fabio Estevam <festevam@denx.de>
Reported-by: Sean Anderson <seanga2@gmail.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Tested-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>