pwm: Move contents of sysfs.c into core.c

pwm: Move contents of sysfs.c into core.c

With the upcoming restructuring having all in a single file simplifies
things a bit. The relevant and somewhat visible changes are:

 - Some dropped prototypes from include/linux/pwm.h that were only
   necessary that core.c has a declaration of the symbols defined in
   sysfs.c. The respective functions are static now.

 - The pwm class now also exists if CONFIG_SYSFS isn't enabled. Having
   CONFIG_SYSFS is not very relevant today, but even without it the
   class and device stuff still provides lifetime tracking.

 - Both files had an initcall, these are merged into a single one now.
   Instead of a big #ifdef block for CONFIG_DEBUG_FS, a single
   if (IS_ENABLED(CONFIG_DEBUG_FS)) is used now. This increases compile
   coverage a bit and is a tad nicer on the eyes.

Link: https://lore.kernel.org/r/9e2d39a5280d7dda5bfc6682a8aef510148635b2.1710670958.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>