feat(esp_eth): add ESP32-S31 EMAC support

Add internal EMAC support for ESP32-S31, including:
- HAL/LL layer: emac_ll, emac_hal, emac_clk, emac_periph for S31
- SOC layer: register structs, soc_caps, peripherals linker script,
  interrupt definitions
- MAC driver: clock source configuration via Kconfig, GPIO/DMA
  adaptations, PHY register defs for clause-45 MDIO
- Ethernet examples updated for S31 (basic, build-test-rules, PTP
  readme)
- Refactored existing ESP32/ESP32-P4 EMAC code for shared patterns
  (emac_clk extraction, struct alignment, periph descriptor cleanup)

components/soc/esp32/register/soc/emac_mac_struct.h

@@ -222,7 +222,8 @@ typedef struct emac_mac_dev_s {
     } emacaddr0high;
     uint32_t emacaddr0low; /*This field contains the lower 32 bits of the first 6-byte MAC address. This is used by the MAC for filtering the received frames and inserting the MAC address in the Transmit Flow Control (Pause) Frames.*/
     emac_mac_addr_t emacaddr[15];    /*Offset: 0x40-0xC0. MAC Address1-15 registers. Each MAC address register contains the high and low 32-bit fields for MAC addresses 1-15.*/
-    uint32_t reserved_10c4; // AN control register
+    uint32_t reserved_10c0;  // AN control register
+    uint32_t reserved_10c4;
     uint32_t reserved_10c8;
     uint32_t reserved_10cc;
     uint32_t reserved_10d0;