dt-bindings: net: rockchip-dwmac: fix rv1126 compatible warning

dt-bindings: net: rockchip-dwmac: fix rv1126 compatible warning

Fix compatible string for RV1126 gmac, and constrain it to
be compatible with Synopsys dwmac 4.20a.

fix below warning
$ make CHECK_DTBS=y rv1126-edgeble-neu2-io.dtb
arch/arm/boot/dts/rv1126-edgeble-neu2-io.dtb: ethernet@ffc40000:
		 compatible: 'oneOf' conditional failed, one must be fixed:
        ['rockchip,rv1126-gmac', 'snps,dwmac-4.20a'] is too long
        'rockchip,rv1126-gmac' is not one of ['rockchip,rk3568-gmac', 'rockchip,rk3588-gmac']

Fixes: b36fe2f43662 ("dt-bindings: net: rockchip-dwmac: add rv1126 compatible")
Reviewed-by: Jagan Teki <jagan@edgeble.ai>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Anand Moon <anand@edgeble.ai>
Link: https://lore.kernel.org/r/20230111172437.5295-1-anand@edgeble.ai
Signed-off-by: Jakub Kicinski <kuba@kernel.org>