pwm: pwm-mediatek: Allocate clk_pwms with devm_kmalloc_array

Switch from devm_kcalloc to devm_kmalloc_array when allocating clk_pwms,
as this structure is being filled right after allocating it, hence
there is no need to zero it out beforehand.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>