[LPC176x] Fix USEABLE_HARDWARE_PWM macro, cleanup multiple LED support (#12129)

Marlin/src/HAL/HAL_LPC1768/fastio.h

 
 #include <Arduino.h>
 
-#define USEABLE_HARDWARE_PWM(pin) useable_hardware_PWM(pin)
+#define USEABLE_HARDWARE_PWM(pin) TRUE // all pins are PWM capable
 
 #define LPC_PIN(pin)            gpio_pin(pin)
 #define LPC_GPIO(port)          gpio_port(port)

Marlin/src/HAL/HAL_LPC1768/main.cpp

 }
 
 void HAL_init() {
+
+  // Support the 4 LEDs some LPC176x boards have
   #if PIN_EXISTS(LED)
     SET_DIR_OUTPUT(LED_PIN);
     WRITE_PIN_CLR(LED_PIN);
-
-    // MKS_SBASE has 3 other LEDs the bootloader uses during flashing. Clear them.
-    SET_DIR_OUTPUT(P1_19);
-    WRITE_PIN_CLR(P1_19);
-    SET_DIR_OUTPUT(P1_20);
-    WRITE_PIN_CLR(P1_20);
-    SET_DIR_OUTPUT(P1_21);
-    WRITE_PIN_CLR(P1_21);
-
+    #if PIN_EXISTS(LED2)
+        SET_DIR_OUTPUT(LED2_PIN);
+        WRITE_PIN_CLR(LED2_PIN);
+      #if PIN_EXISTS(LED3)
+          SET_DIR_OUTPUT(LED3_PIN);
+          WRITE_PIN_CLR(LED3_PIN);
+        #if PIN_EXISTS(LED4)
+            SET_DIR_OUTPUT(LED4_PIN);
+            WRITE_PIN_CLR(LED4_PIN);
+        #endif
+      #endif
+    #endif
     // Flash status LED 3 times to indicate Marlin has started booting
     for (uint8_t i = 0; i < 6; ++i) {
       TOGGLE(LED_PIN);
       delay(100);
     }
   #endif
+
   //debug_frmwrk_init();
   //_DBG("\n\nDebug running\n");
   // Initialise the SD card chip select pins as soon as possible

Marlin/src/HAL/HAL_LPC1768/watchdog.cpp

 void watchdog_reset() {
   WDT_Feed();
   #if DISABLED(PINS_DEBUGGING) && PIN_EXISTS(LED)
-    TOGGLE(LED_PIN);  // heart beat indicator
+    TOGGLE(LED_PIN);  // heartbeat indicator
   #endif
 }
 

Marlin/src/pins/pins_MKS_SBASE.h

 #define PIN_P0_03          P0_03   // AUX1 (Interrupt Capable/ADC/Serial Port 0)
 */
 
-#define LED_PIN            P1_18   // LED2 P1_19, LED3 P1_20, LED4 P1_21
+#define LED_PIN            P1_18   // Used as a status indicator
+#define LED2_PIN           P1_19
+#define LED3_PIN           P1_20
+#define LED4_PIN           P1_21
 
 //
 // Servo pin