lib: Add Atmel SAMD21 cmsis headers

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

lib/samd21/samd21a/include/system_samd21.h

@@ -0,0 +1,48 @@
+/**
+ * \file
+ *
+ * \brief Low-level initialization functions called upon chip startup
+ *
+ * Copyright (c) 2018 Microchip Technology Inc.
+ *
+ * \asf_license_start
+ *
+ * \page License
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the Licence at
+ * 
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * 
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an AS IS BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * \asf_license_stop
+ *
+ */
+
+#ifndef _SYSTEM_SAMD21_H_INCLUDED_
+#define _SYSTEM_SAMD21_H_INCLUDED_
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <stdint.h>
+
+extern uint32_t SystemCoreClock;   /*!< System Clock Frequency (Core Clock)  */
+
+void SystemInit(void);
+void SystemCoreClockUpdate(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* SYSTEM_SAMD21_H_INCLUDED */