driver: pwm: npcx: Add output open drain support

NPCX PWM supports output buffet select to push-pull or open-drain. Add
output buffer select option 'drive-open-drain' in devicetree for NPCX
PWM. If set, the PWM output will be configured as open-drain. If not
set, defaults to push-pull.

Signed-off-by: Mulin Chao <mlchao@nuvoton.com>
Signed-off-by: Wealian Liao <WHLIAO@nuvoton.com>