pwm: sti: Implement .apply() callback

[ Upstream commit b2e60b32 ]

To eventually get rid of all legacy drivers convert this driver to the
modern world implementing .apply().
This just pushed a variant of pwm_apply_legacy() into the driver that was
slightly simplified because the driver doesn't provide a .set_polarity()
callback.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
Stable-dep-of: 5f623835 ("pwm: sti: Fix capture for st,pwm-num-chan < st,capture-num-chan")
Signed-off-by: Sasha Levin <sashal@kernel.org>