pwm: imx27: Ensure clocks being on iff the PWM is on

Up to now the .probe() function didn't enable clocks and relied on the
core to call the .get_state() callback to have the clock running. The
latter enabled the needed clocks and kept them running if the PWM wass
enabled.

This only works correctly if the .get_state() callback is called exactly
once and this single call happens before unused clocks are disabled by
the clk core.

The former wasn't true for a short period while commit 01ccf903
("pwm: Let pwm_get_state() return the last implemented state") applied
and not reverted yet and might become wrong in the future.

The latter isn't true any more since commit cfc4c189 ("pwm: Read
initial hardware state at request time") which results in a running PWM
being stopped at boot time if for example the consumer lives in a kernel
module that is only loaded after the clk core disabled unused clocks.

So ensure .probe() is left with the clocks on if the PWM is running and
.get_state() disables everything it enabled.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>