net: stmmac: platform: Add snps,dwmac-5.20 IP compatible string

Add "snps,dwmac-5.20" compatible string for 5.20 version that can avoid
to define some platform data in the glue layer.

Tested-by: Tommaso Merciai <tomm.merciai@gmail.com>
Signed-off-by: Emil Renner Berthing <kernel@esmil.dk>
Signed-off-by: Samin Guo <samin.guo@starfivetech.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>