drivers: pwm: mcux_ftm: allow to select clock source

FTM internal counter can be clocked by one of three clock sources
independent of the module bus clock. This patch introduces a DT property
to perform the clock selection from DT.

DT sources are updated to keep the current clock selection for all boards,
with exception of ucans32k1sic board which is migrated to use system
clock by default, as this seems to be a better choice for most cases.
Some PWM LED samples require slower clock so overlays are added for
those cases.

Signed-off-by: Manuel Argüelles <manuel.arguelles@nxp.com>

dts/bindings/pwm/nxp,kinetis-ftm-pwm.yaml

@@ -1,4 +1,4 @@
-# Copyright (c) 2017, NXP
+# Copyright 2017, 2024 NXP
 # SPDX-License-Identifier: Apache-2.0
 
 description: Kinetis FTM PWM
@@ -14,6 +14,25 @@ properties:
   pinctrl-0:
     required: true
 
+  clock-source:
+    type: string
+    required: true
+    enum:
+      - "system"
+      - "fixed"
+      - "external"
+    description: |
+      Select one of three possible clock sources for the FTM counter:
+        * system: it's the bus interface clock driving the FTM module. Usually
+          provides higher timer resolution than the other two clock sources.
+        * fixed: it's a fixed clock defined by chip integration.
+        * external: it's a clock that can be accessed externally to the chip and
+          passes through a sychronizer clocked by the FTM bus interface clock.
+
+      This clock source selection is independent of the bus interface clock
+      driving the FTM module. Refer to the chip specific documentation for
+      further information.
+
 pwm-cells:
   - channel
   # period in terms of nanoseconds