mfd: stm32: Enable regmap fast_io for stm32-lptimer

Because stm32-lptimer need to write in registers in interrupt context
enable regmap fast_io to use a spin_lock to protect registers access
rather than a mutex.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>