dt-bindings: net: cdns,macb: allow tsu_clk without tx_clk

dt-bindings: net: cdns,macb: allow tsu_clk without tx_clk

Allow providing tsu_clk without a tx_clk as both are optional.

This is about relaxing unneeded constraints. It so happened that in the
past HW that needed a tsu_clk always needed a tx_clk.

Fixes: 4e5b6de1f46d ("dt-bindings: net: cdns,macb: Convert to json-schema")
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
Link: https://patch.msgid.link/20250923-macb-fixes-v6-1-772d655cdeb6@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>