pwm: atmel: Simplify using devm functions

For all resources used by the driver there is a devm variant to allocate
these. This simplifies the error path in the probe callback and allows
to drop the remove callback.

While at it also use dev_err_probe() to compact returning an error.

With the remove callback gone, there is no user of driver data left, so
the call to platform_set_drvdata() can also be dropped.

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