feat(uart): allow pins_arduino.h to define esp32-p4 uart pins

ESP32-P4 has UART default pins only for UART0 and UART1.
This PR allows the board definition from pins_arduino.h to define RX2 ... RX4 and TX2 ... TX4 if necessary.
It also solves the issue of begin(baud) with no pins for UART2...4 by just sending a error message and returning.

Author: SuGlider

cores/esp32/HardwareSerial.cpp

@@ -341,14 +341,63 @@ void HardwareSerial::begin(unsigned long baud, uint32_t config, int8_t rxPin, in
       case UART_NUM_2:
         if (rxPin < 0 && txPin < 0) {
           // do not change RX2/TX2 if it has already been set before
+#ifdef RX2
           rxPin = _rxPin < 0 ? (int8_t)RX2 : _rxPin;
+#else
+          rxPin = _rxPin;
+#endif
+#ifdef TX2
           txPin = _txPin < 0 ? (int8_t)TX2 : _txPin;
+#else
+          txPin = _txPin;
+#endif
+        }
+        break;
+#endif
+#if SOC_UART_HP_NUM > 3  // may save some flash bytes...
+      case UART_NUM_3:
+        if (rxPin < 0 && txPin < 0) {
+          // do not change RX2/TX2 if it has already been set before
+#ifdef RX3
+          rxPin = _rxPin < 0 ? (int8_t)RX3 : _rxPin;
+#else
+          rxPin = _rxPin;
+#endif
+#ifdef TX3
+          txPin = _txPin < 0 ? (int8_t)TX3 : _txPin;
+#else
+          txPin = _txPin;
+#endif
+        }
+        break;
+#endif
+#if SOC_UART_HP_NUM > 4  // may save some flash bytes...
+      case UART_NUM_4:
+        if (rxPin < 0 && txPin < 0) {
+          // do not change RX2/TX2 if it has already been set before
+#ifdef RX4
+          rxPin = _rxPin < 0 ? (int8_t)RX4 : _rxPin;
+#else
+          rxPin = _rxPin;
+#endif
+#ifdef TX4
+          txPin = _txPin < 0 ? (int8_t)TX4 : _txPin;
+#else
+          txPin = _txPin;
+#endif
         }
         break;
 #endif
     }
   }
 
+  // if no RX/TX pins are defined, it will not start the UART driver
+  if (rxPin < 0 && txPin < 0) {
+    log_e("No RX/TX pins defined. Please set RX/TX pins.");
+    HSERIAL_MUTEX_UNLOCK();
+    return;
+  }
+  
   // IDF UART driver keeps Pin setting on restarting. Negative Pin number will keep it unmodified.
   // it will detach previous UART attached pins