dt-bindings: pwm: Update STM32 timers clock names

Clock name has been updated during driver/DT binding review:
https://lkml.org/lkml/2016/12/13/718
Update DT binding doc to reflect this.

Fixes: cd9a99c2 (dt-bindings: pwm: Add STM32 bindings)
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

Documentation/devicetree/bindings/pwm/pwm-stm32.txt

@@ -24,7 +24,7 @@ Example:
 		compatible = "st,stm32-timers";
 		reg = <0x40010000 0x400>;
 		clocks = <&rcc 0 160>;
-		clock-names = "clk_int";
+		clock-names = "int";
 
 		pwm {
 			compatible = "st,stm32-pwm";