pwm: mediatek: Fix clock control issue

In order to save some power, do not prepare the top and main clocks
during mtk_pwm_probe(). Instead, prepare the clocks only when necessary
and also make sure to enable the clocks to match the semantics of the
common clock framework.

While at it, don't explicitly disable all PWM channels in ->remove()
because all users should have done that already.

Signed-off-by: Zhi Mao <zhi.mao@mediatek.com>
Acked-by: John Crispin <john@phrozen.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>