pwm: pwm-tiehrpwm: 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-tiehrpwm.c:562:12: warning: 'ehrpwm_pwm_suspend' defined but not used [-Wunused-function]
drivers/pwm/pwm-tiehrpwm.c:580:12: warning: 'ehrpwm_pwm_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>