boards/waspmote: Fix timer config

- Set XTIMER_HZ to something that is actually possible to generate with one
  of the available clock dividers from the core frequency
- Use xtimer_on_ztimer if xtimer is used and not ztimer_xtimer_compat is used
    - This is needed because xtimer is simply not compatible with any of the
      possible clock frequencies of this board

boards/waspmote-pro/Makefile.dep

@@ -1,3 +1,5 @@
 #ifneq (,$(filter saul_default,$(USEMODULE)))
 #  USEMODULE += saul_gpio
 #endif
+
+include $(RIOTMAKE)/boards/ztimer_only.dep.mk

boards/waspmote-pro/include/board.h

@@ -159,13 +159,26 @@ extern "C" {
 
 /**
  * @name    xtimer configuration values
+ * @warning This configuration is not actually compatible with xtimer. Sadly,
+ *          no compatible clock frequency can be generated with the given core
+ *          frequency
  * @{
  */
 #define XTIMER_WIDTH                (16)
-#define XTIMER_HZ                   (62500UL)
+#define XTIMER_HZ                   (230400LU)
 #define XTIMER_BACKOFF              (40)
 /** @} */
 
+/**
+ * @name    ztimer configuration values
+ * @{
+ */
+#define CONFIG_ZTIMER_USEC_TYPE     ZTIMER_TYPE_PERIPH_TIMER
+#define CONFIG_ZTIMER_USEC_DEV      (TIMER_DEV(0))
+#define CONFIG_ZTIMER_USEC_FREQ     (230400LU)
+#define CONFIG_ZTIMER_USEC_WIDTH    (16)
+/** @} */
+
 /**
  * @brief   Initialize board specific hardware, including clock, LEDs and std-IO
  */

makefiles/boards/ztimer_only.dep.mk

@@ -0,0 +1,11 @@
+# Include this for boards whose timer periph cannot generate a clock frequency
+# suitable for xtimer with the available clock sources and dividers.
+# This will use ztimer to perform the required frequency conversion.
+# By default, xtimer is still used with ztimer as backed, unless
+# ztimer_xtimer_compat is used.
+
+ifneq (,$(filter xtimer,$(USEMODULE)))
+  ifeq (,$(filter ztimer_xtimer_compat,$(USEMODULE)))
+    USEMODULE += xtimer_on_ztimer
+  endif
+endif