pwm: sti: Simplify probe function using devm functions

pwm: sti: Simplify probe function using devm functions

Instead of of_clk_get_by_name() use devm_clk_get_prepared() which has
several advantages:

 - Combines getting the clock and a call to clk_prepare(). The latter
   can be dropped from sti_pwm_probe() accordingly.
 - Cares for calling clk_put() which is missing in both probe's error
   path and the remove function.
 - Cares for calling clk_unprepare() which can be dropped from the error
   paths and the remove function. (Note that not all error path got this
   right.)

With additionally using devm_pwmchip_add() instead of pwmchip_add() the
remove callback can be dropped completely. With it the last user of
platform_get_drvdata() goes away and so platform_set_drvdata() can be
dropped from the probe function, too.

Fixes: 378fe115d19d ("pwm: sti: Add new driver for ST's PWM IP")
Link: https://lore.kernel.org/r/81f0e1d173652f435afda6719adaed1922fe059a.1710068192.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>