dt-bindings: ethernet: eswin: add optional TXD and RXD delay register offsets

dt-bindings: ethernet: eswin: add optional TXD and RXD delay register offsets

Document two optional cells in eswin,hsp-sp-csr for the TXD and RXD
delay control register offsets.

These registers are used by the driver to clear any residual delay
configuration left by the bootloader, ensuring that MAC-side RGMII delay
settings are applied solely according to the kernel configuration.

Add a reference to the EIC7700X SoC Technical Reference Manual for
background information about the HSP CSR block.

Fixes: 888bd0eca93c ("dt-bindings: ethernet: eswin: Document for EIC7700 SoC")
Signed-off-by: Zhi Li <lizhi2@eswincomputing.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20260518022023.427-1-lizhi2@eswincomputing.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>