cpu/sam0_common: provide function to switch voltage regulator

Add a fucntion to switch between LDO and Buck concerter to provide the
internal CPU voltage.
The Buck Converter is not compatible with internal fast oscillators (DFLL, DPLL)
and requires an inductivity to be present on the board.

cpu/sam0_common/include/periph_cpu_common.h

@@ -392,6 +392,39 @@ static inline void sam0_cortexm_sleep(int deep)
  */
 void gpio_disable_mux(gpio_t pin);
 
+/**
+ * @brief   Available voltage regulators on the supply controller.
+ */
+typedef enum {
+    SAM0_VREG_LDO,  /*< LDO, always available but not very power efficient */
+    SAM0_VREG_BUCK  /*< Buck converter, efficient but may clash with internal
+                        fast clock generators (see errata sheets) */
+} sam0_supc_t;
+
+/**
+ * @brief       Switch the internal voltage regulator used for generating the
+ *              internal MCU voltages.
+ *              Available options are:
+ *
+ *               - LDO: not very efficient, but will always work
+ *               - BUCK converter: Most efficient, but incompatible with the
+ *                 use of DFLL or DPLL.
+ *                 Please refer to the errata sheet, further restrictions may
+ *                 apply depending on the MCU.
+ *
+ * @param[in]   src
+ */
+static inline void sam0_set_voltage_regulator(sam0_supc_t src)
+{
+#ifdef REG_SUPC_VREG
+    SUPC->VREG.bit.SEL = src;
+    while (!SUPC->STATUS.bit.VREGRDY) {}
+#else
+    (void) src;
+    assert(0);
+#endif
+}
+
 /**
  * @brief   Returns the frequency of a GCLK provider.
  *