cpu/saml1x: enable buck voltage regulator

Switch from the on-chip LDO to the on-chip buck voltage regulator.

cpu/saml1x/cpu.c

@@ -30,6 +30,16 @@
 #define _NVMCTRL NVMCTRL
 #endif
 
+/* As long as FDPLL is not used, we can default to
+ * always using the buck converter.
+ *
+ * An external inductor needs to be present on the board,
+ * so the feature can only be enabled by the board configuration.
+ */
+#ifndef USE_VREG_BUCK
+#define USE_VREG_BUCK (0)
+#endif
+
 static void _gclk_setup(int gclk, uint32_t reg)
 {
     GCLK->GENCTRL[gclk].reg = reg;
@@ -97,6 +107,11 @@ void cpu_init(void)
     /* initialize the Cortex-M core */
     cortexm_init();
 
+    /* not compatible with 96 MHz FDPLL */
+    if (USE_VREG_BUCK) {
+        sam0_set_voltage_regulator(SAM0_VREG_BUCK);
+    }
+
     /* turn on only needed APB peripherals */
     MCLK->APBAMASK.reg = MCLK_APBAMASK_MCLK
                          | MCLK_APBAMASK_OSCCTRL