dts/bindings/clock: st-stm32(wb)-rcc: Clarify 'clock-frequency' meaning

Rework bindings documentation to clearly illustrate the role of ahb
(and cpu1) prescaler which defines the actual core clock frequency,
and not only a bus frequency.

Signed-off-by: Erwan Gouriou <[email protected]>

Author: erwango

dts/bindings/clock/st,stm32-rcc.yaml

@@ -10,15 +10,16 @@ description: |
 
   System clock source should be selected amongst the clock nodes available in "clocks"
   node (typically 'clk_hse, clk_hsi', 'pll', ...).
-  As part of this node configuration, SYSCLK frequency should also be defined, using
-  "clock-frequency" property.
-  Last, bus clocks (typically HCLK, PCLK1, PCLK2) should be configured using matching
+  Core clock frequency should also be defined, using "clock-frequency" property.
+  Note:
+          Core clock frequency  = SYSCLK / AHB prescaler
+  Last, peripheral bus clocks (typically PCLK1, PCLK2) should be configured using matching
   prescaler properties.
   Here is an example of correctly configured rcc node:
   &rcc {
-           clocks = <&pll>;  /* Set pll as SYSCLK source */
-           clock-frequency = <DT_FREQ_M(80)>; /* SYSCLK runs at 80MHz */
-           ahb-prescaler = <1>;
+           clocks = <&pll>;  /* Select 80MHz pll as SYSCLK source */
+           ahb-prescaler = <2>;
+           clock-frequency = <DT_FREQ_M(40)>; /* = SYSCLK / AHB prescaler */
            apb1-presacler = <1>;
            apb2-presacler = <1>;
   }
@@ -67,8 +68,8 @@ properties:
         - 256
         - 512
       description: |
-          AHB prescaler. Sets a HCLK frequency (feeding Cortex-M Systick)
-          lower than SYSCLK frequency (actual core frequency).
+          AHB prescaler. Defines actual core clock frequency (HCLK)
+          based on system frequency input.
           The HCLK clocks CPU, AHB, memories and DMA.
 
     apb1-prescaler:

dts/bindings/clock/st,stm32wb-rcc.yaml

@@ -32,8 +32,8 @@ properties:
         - 256
         - 512
       description: |
-          CPU1 prescaler. Sets a HCLK1 frequency (feeding Cortex-M Systick)
-          lower than SYSCLK frequency (actual core frequency).
+          CPU1 prescaler. Sets a HCLK1 frequency (Core frequency)
+          lower than SYSCLK frequency.
           The HCLK1 clocks CPU1, AHB1, AHB2, AHB3 and SRAM1.
 
     cpu2-prescaler: