dt-bindings: ethernet: eswin: fix hsp-sp-csr backward compatibility

dt-bindings: ethernet: eswin: fix hsp-sp-csr backward compatibility

Commit c36069c6f46c ("dt-bindings: ethernet: eswin: add optional TXD and
RXD delay register offsets") added two optional cells to eswin,hsp-sp-csr
but omitted minItems: 4.

As a result, dt-schema implicitly required all 6 cells, which broke
backward compatibility with existing 4-cell device trees.

Add minItems: 4 to preserve backward compatibility.

Fixes: c36069c6f46c ("dt-bindings: ethernet: eswin: add optional TXD and RXD delay register offsets")
Reported-by: Sashiko AI <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260519022334.35742C2BCB7@smtp.kernel.org/
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Zhi Li <lizhi2@eswincomputing.com>
Link: https://patch.msgid.link/20260602014528.2076-1-lizhi2@eswincomputing.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>