dts: bindings: lptim stm32 has a prescaler entry for the lptim clock

This is a new parameter to divide the LPTIM input clock
by a prescaler, changing the max reachable timeout of the LP timer.
It will divide the LPTIM input clock by 1 (reset value) up to 128.
The lptim configuration register is written with a 3bit value.

Signed-off-by: Francois Ramu <francois.ramu@st.com>

dts/bindings/timer/st,stm32-lptim.yaml

@@ -14,6 +14,32 @@ include:
       - st,countermode
 
 properties:
+  st,prescaler:
+    type: int
+    default: 1
+    description: |
+        Clock divider at the input of the lptimer clock, default reset value is 'not divided'.
+
+        Prescaler allows to achieve higher LPTIM timeout (up to 256s when lptim clocked by LSE)
+        and consequently higher core sleep durations, but impacts the tick precision.
+        To be used with caution.
+        CONFIG_SYS_CLOCK_TICKS_PER_SEC could be used to tune tick duration and gain precision,
+        at kernel timing granularity cost.
+
+        For example, when LPTIM is clocked by the LSE (32768Hz) and st,prescaler = <32>:
+        LPTIM global timeout is 64 seconds with an increment of 0.97 ms.
+        Using CONFIG_SYS_CLOCK_TICKS_PER_SEC = 4096, tick = 0.24 ms, LPTIM precision is 4 ticks.
+        Using CONFIG_SYS_CLOCK_TICKS_PER_SEC = 1024, tick = 0.97 ms, LPTIM precision is 1 ticks.
+    enum:
+      - 1
+      - 2
+      - 4
+      - 8
+      - 16
+      - 32
+      - 64
+      - 128
+
   st,static-prescaler:
     type: boolean
     description: |