stmmac: platform: add "snps, dwmac-5.10a" IP compatible string

Add "snps,dwmac-5.10a" compatible string for 5.10a version that can
avoid to define some plat data in glue layer.

Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>