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>