pwm: stm32: Enforce dependency on CONFIG_MFD_STM32_TIMERS

When compile-testing the PWM driver without also enabling the
stm32_timers MFD, we run into a link error:

	drivers/pwm/pwm-stm32.o: In function `stm32_pwm_raw_capture.isra.6':
	pwm-stm32.c:(.text+0xcb0): undefined reference to `stm32_timers_dma_burst_read'

We don't need the '|| COMPILE_TEST' here, since stm32_timers itself
can be built with CONFIG_COMPILE_TEST on all architectures, so we do
get the coverage through allmodconfig and randconfig builds even
when we make it a hard dependency.

Fixes: 7edf7369

 ("pwm: Add driver for STM32 plaftorm")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>