pwm: mediatek: Always use bus clock

The MediaTek PWM IP can sometimes use the 26 MHz source clock to
generate the PWM signal, but the driver currently assumes that we always
use the PWM bus clock to generate the PWM signal.

This commit modifies the PWM driver in order to force the PWM IP to
always use the bus clock as source clock.

I do not have the datasheet of all the MediaTek SoC, so I don't know if
the register to choose the source clock is present in all the SoCs or
only in subset. As a consequence I made this change optional by using a
platform data paremeter to says whether this register is supported or
not. On all the SoCs I don't have the datasheet (MT2712, MT7622, MT7623,
MT7628, MT7629) I kept the behavior to be the same as before this
change.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>