pwm: tiecap: add CONFIG_PM_SLEEP to ecap_pwm_{save,restore}_context()

ecap_pwm_save_context() and ecap_pwm_restore_context() are only used
when CONFIG_PM_SLEEP is selected.

drivers/pwm/pwm-tiecap.c:293:13: warning: 'ecap_pwm_save_context' defined but not used [-Wunused-function]
drivers/pwm/pwm-tiecap.c:302:13: warning: 'ecap_pwm_restore_context' defined but not used [-Wunused-function]

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