dt-bindings: mfd: 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: 8f9359c6 (dt-bindings: mfd: Add bindings for STM32 Timers driver)
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>