Add bindings information for STM32 Timers

version 6:
- rename stm32-gtimer to stm32-timers
- change compatible
- add description about the IPs

version 2:
- rename stm32-mfd-timer to stm32-gptimer
- only keep one compatible string
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>