pwm: tiehrpwm: don't build PM related functions when not needed

Fixes following warnings on AM335X with no PM_SLEEP

drivers/pwm/pwm-tiehrpwm.c:534:13: warning: 'ehrpwm_pwm_save_context' defined but not used [-Wunused-function]
drivers/pwm/pwm-tiehrpwm.c:548:13: warning: 'ehrpwm_pwm_restore_context' defined but not used [-Wunused-function]

Signed-off-by: Wolfram Sang <wsa@sang-engineering.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>