boards/xg23-pk6068a: remove HFXO_FREQ and LFXO_FREQ

Prior to the use of Gecko SDK 4.5, RIOT shipped with patched vendor
files, such as `system_xxx.c`. These system files included
`periph_config.h`, which allowd one to redefine `HFXO_FREQ` and
`LFXO_FREQ`.

This was not intended, and other EFM32-based boards documented this to
redefine this using compiler defines. After all, a board configuration
is considered to be somewhat pre-defined and static.

With the new Gecko SDK 4.5, this (unintended) feature got lost for the
`xg23-pk6068a` board. To bring it in-line with the other boards,
remove the defines, and document how it should be used.

Within the codebase, `LFXO_FREQ` was used for ztimer configuration.
Similar to the other boards, this has been replaced with a static
value.

Behind the scenes, the vendor `system_xxx.c` file will still use the
same values.

Author: basilfx

boards/xg23-pk6068a/doc.md

@@ -94,6 +94,23 @@ PIN 1 is the bottom-left contact when the header faces  you horizontally.
 | LCD driver                    | LS013B7DH03 | yes       | Sharp Low Power Memory LCD via the U8g2 package                |
 | Temperature + humidity sensor | Si7021      | yes       | Silicon Labs Temperature + Humidity sensor                     |
 
+## Board configuration
+
+### Clock selection
+There are several clock sources that are available for the different
+peripherals. You are advised to read [AN0004.0](https://www.silabs.com/documents/public/application-notes/an0004.0-efm32-cmu.pdf)
+to get familiar with the different clocks.
+
+| Source | Internal | Speed      | Comments                           |
+|--------|----------|------------|------------------------------------|
+| HFXO   | No       | 39 MHz     |                                    |
+| LFXO   | No       | 32.768 kHz |                                    |
+
+It is important that the clock speeds are known to the code, for proper
+calculations of speeds and baud rates. If the HFXO or LFXO are different from
+the speeds above, ensure to pass `HFXO_FREQ=freq_in_hz` and
+`LFXO_FREQ=freq_in_hz` to your compiler defines.
+
 ## Flashing the device
 To flash, [SEGGER JLink](https://www.segger.com/jlink-software.html) is
 required.

boards/xg23-pk6068a/include/board.h

@@ -36,7 +36,7 @@ extern "C" {
  * @{
  */
 #define CONFIG_ZTIMER_LPTIMER_DEV            TIMER_DEV(1)
-#define CONFIG_ZTIMER_LPTIMER_FREQ           LFXO_FREQ
+#define CONFIG_ZTIMER_LPTIMER_FREQ           (32768UL)
 #define CONFIG_ZTIMER_LPTIMER_WIDTH          24
 #define CONFIG_ZTIMER_LPTIMER_BLOCK_PM_MODE  EFM32_PM_MODE_EM3
 /** @} */

boards/xg23-pk6068a/include/periph_conf.h

@@ -28,10 +28,7 @@ extern "C" {
  * @name    Clock configuration
  * @{
  */
-#define HFXO_FREQ           (39000000UL)
 #define CMU_HFXOINIT        CMU_HFXOINIT_DEFAULT
-
-#define LFXO_FREQ           (32768UL)
 #define CMU_LFXOINIT        CMU_LFXOINIT_DEFAULT
 
 static const clk_mux_t clk_mux_config[] = {