pwm: mediatek: Allocate the clks array dynamically

Instead of using fixed size of arrays, allocate the memory for them
based on the number of PWMs specified for each SoC generation.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Sam Shih <sam.shih@mediatek.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>