pwm: sysfs: Add suspend/resume support

According to the Documentation/pwm.txt, all PWM consumers should have
power management. Since this sysfs interface is one of consumers so that
this patch adds suspend/resume support.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
[thierry.reding@gmail.com: fix build warnings for !PM]
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>