pwm: pwm-tipwmss: add CONFIG_PM_SLEEP to suspend/resume functions

This patch adds CONFIG_PM_SLEEP to suspend/resume functions to fix
the following build warning when CONFIG_PM_SLEEP is not selected.

drivers/pwm/pwm-tipwmss.c:104:12: warning: 'pwmss_suspend' defined but not used [-Wunused-function]
drivers/pwm/pwm-tipwmss.c:113:12: warning: 'pwmss_resume' defined but not used [-Wunused-function]

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>