net: dwc_eth_qos: update the compatible supported for STM32

Update the compatible associated with the STM32 MPU glue
in the DWC ethernet driver.

The supported compatible is the specific "st,stm32mp1-dwmac"
as indicated in Linux binding
Documentation/devicetree/bindings/net/stm32-dwmac.txt
and not the "snps,dwmac-4.20a" only used to the select IP
version.

This glue is implemented in Linux kernel in:
drivers/net/ethernet/stmicro/stmmac/dwmac-stm32.c

For information in stm32mp151.dtsi, the 2 compatibles are
supported:

ethernet0: ethernet@5800a000 {
	compatible = "st,stm32mp1-dwmac", "snps,dwmac-4.20a";
	...
};

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>