pwm: pwm-tiehrpwm: pinctrl support

Enable pinctrl for pwm-tiehrpwm if pinctrl driver available, else
bail out with warning message.

Signed-off-by: Philip, Avinash <avinashphilip@ti.com>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>