mips: dts: lantiq: danube: add missing burst length property

The upstream dts lacks the lantiq,{rx/tx}-burst-length property. Other
issues were also fixed:
arch/mips/boot/dts/lantiq/danube_easy50712.dtb: etop@e180000 (lantiq,etop-xway): 'interrupt-names' is a required property
	from schema $id: http://devicetree.org/schemas/net/lantiq,etop-xway.yaml#
arch/mips/boot/dts/lantiq/danube_easy50712.dtb: etop@e180000 (lantiq,etop-xway): 'lantiq,tx-burst-length' is a required property
	from schema $id: http://devicetree.org/schemas/net/lantiq,etop-xway.yaml#
arch/mips/boot/dts/lantiq/danube_easy50712.dtb: etop@e180000 (lantiq,etop-xway): 'lantiq,rx-burst-length' is a required property
	from schema $id: http://devicetree.org/schemas/net/lantiq,etop-xway.yaml#



Fixes: 14d4e308 ("net: lantiq: configure the burst length in ethernet drivers")
Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Acked-by: Jakub Kicinski <kuba@kernel.org>