pwm: pwm-tipwmss: Remove all runtime PM gets/puts

Remove all pm_runtime_get_sync() and pm_runtime_put_sync() call as well
as the dummy pm_ops from the pwm-tipwmss driver. No registers are being
modified. The runtime PM still needs to be enabled, so that the runtime
PM framework can take care of enabling/disabling the PWMSS clock when
submodules of PWMSS (ECAP or EHRPWM) call runtime PM APIs. With this
change PWMSS clock goes to idle when none of the submodules are in use.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>