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 <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Anand Moon <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/devicetree/bindings/net/rockchip-dwmac.yaml

···
               - rockchip,rk3368-gmac
               - rockchip,rk3399-gmac
               - rockchip,rv1108-gmac
-              - rockchip,rv1126-gmac
       - items:
           - enum:
               - rockchip,rk3568-gmac
               - rockchip,rk3588-gmac
+              - rockchip,rv1126-gmac
           - const: snps,dwmac-4.20a
 
   clocks: