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: 888bd0eca9 ("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>

Documentation/devicetree/bindings/net/eswin,eic7700-eth.yaml

@@ -73,6 +73,15 @@ properties:
       HSP CSR is to control and get status of different high-speed peripherals
       (such as Ethernet, USB, SATA, etc.) via register, which can tune
       board-level's parameters of PHY, etc.
+
+      Additional background information about the High-Speed Subsystem
+      and the HSP CSR block is available in Chapter 10 ("High-Speed Interface")
+      of the EIC7700X SoC Technical Reference Manual, Part 4
+      (EIC7700X_SoC_Technical_Reference_Manual_Part4.pdf). The manual is
+      publicly available at
+      https://github.com/eswincomputing/EIC7700X-SoC-Technical-Reference-Manual/releases
+
+      This reference is provided for background information only.
     $ref: /schemas/types.yaml#/definitions/phandle-array
     items:
       - items:
@@ -82,6 +91,8 @@ properties:
           - description: Offset of AXI clock controller Low-Power request
                          register
           - description: Offset of register controlling TX/RX clock delay
+          - description: Optional offset of register controlling TXD delay
+          - description: Optional offset of register controlling RXD delay
 
 required:
   - compatible
@@ -116,7 +127,7 @@ examples:
         reset-names = "stmmaceth";
         rx-internal-delay-ps = <200>;
         tx-internal-delay-ps = <200>;
-        eswin,hsp-sp-csr = <&hsp_sp_csr 0x100 0x108 0x118>;
+        eswin,hsp-sp-csr = <&hsp_sp_csr 0x100 0x108 0x118 0x114 0x11c>;
         snps,axi-config = <&stmmac_axi_setup>;
         snps,aal;
         snps,fixed-burst;