rk3399: enable the pwm2/3 pinctrl in board init

There is no interrupt line for each PWM which used by pinctrl to get the
periph_id, so it's not able to enable the default pinctrl setting by pinctrl
framework, let's enable it at board_init().

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>