samd/clock_config: Set up the clock configuration.

Clock settings:
- GCLK0: 48 MHz (SAMD21) or 120 MHz(SAMD51).
- GCLK1: 32768 Hz for driving the PLL.
- GCLK2: 48 MHz for tzhe peripheral clock.
- GCLK3: 1 MHz (SAMD21) or 8 MHz (SAMD51) for the µs ticks timer.
- GCLK8: 1 kHz for WDT (SAMD21 only).

If a 32 kHz crystal is present, it will be used as clock source.  Otherwise
the DFLL48M in open-loop mode is used.

GCLK0 for SAM51 can be changed between 48 MHz and 200 MHz.  The specified
range is 96 MHz - 120 MHz.

ports/samd/clock_config.h

@@ -0,0 +1,33 @@
+/*
+ * This file is part of the MicroPython project, http://micropython.org/
+ *
+ * This file provides functions for configuring the clocks.
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2022 Robert Hammelrath
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+void init_clocks(uint32_t cpu_freq);
+void set_cpu_freq(uint32_t cpu_freq);
+uint32_t get_cpu_freq(void);
+uint32_t get_apb_freq(void);
+void enable_sercom_clock(int id);