Reorganize HAL (#14832)

Marlin/src/HAL/HAL_SAMD51/HAL.h

@@ -25,9 +25,9 @@
 #include "../shared/Marduino.h"
 #include "../shared/math_32bit.h"
 #include "../shared/HAL_SPI.h"
-#include "fastio_SAMD51.h"
-#include "watchdog_SAMD51.h"
-#include "HAL_timers_SAMD51.h"
+#include "fastio.h"
+#include "watchdog.h"
+#include "timers.h"
 
 #ifdef ADAFRUIT_GRAND_CENTRAL_M4
   #include "MarlinSerial_AGCM4.h"

Marlin/src/HAL/HAL_SAMD51/Servo.cpp

@@ -33,7 +33,7 @@
 #include "../shared/servo.h"
 #include "../shared/servo_private.h"
 #include "SAMD51.h"
-#include "HAL_timers_SAMD51.h"
+#include "timers.h"
 
 #define __TC_GCLK_ID(t)         TC##t##_GCLK_ID
 #define _TC_GCLK_ID(t)          __TC_GCLK_ID(t)

Marlin/src/HAL/HAL_SAMD51/timers.cpp

@@ -25,7 +25,7 @@
 // Includes
 // --------------------------------------------------------------------------
 #include "../../inc/MarlinConfig.h"
-#include "HAL_timers_SAMD51.h"
+#include "timers.h"
 
 // --------------------------------------------------------------------------
 // Local defines

Marlin/src/HAL/HAL_SAMD51/watchdog.cpp

@@ -25,7 +25,7 @@
 
 #if ENABLED(USE_WATCHDOG)
 
-  #include "watchdog_SAMD51.h"
+  #include "watchdog.h"
 
   void watchdog_init(void) {
     // The low-power oscillator used by the WDT runs at 32,768 Hz with