mfd: stm32-timers: Use devm_of_platform_populate()

Use devm_of_platform_populate() instead of of_platform_populate()
and suppress stm32_timers_remove() which become useless.

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