leds: pwm: Remove platform_data support

Since commit 141f15c6 ("leds: pwm: remove header") that platform
interface is not usable from outside and there seems to be no in tree
user anymore.  All in-tree users of the leds-pwm driver seem to use DT
currently.  Getting rid of the old platform interface allows the
leds-pwm driver to switch over from 'devm_led_classdev_register()' to
'devm_led_classdev_register_ext()'.

Signed-off-by: Alexander Dahl <post@lespocky.de>
Cc: Denis Osterland-Heim <denis.osterland@diehl.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Signed-off-by: Pavel Machek <pavel@ucw.cz>