pinctrl: npcx: config pwm open-drain without enabling STORE_REG

Config pwm open-drain mode without enabling STORE_REG. This CL
collects all active PWM's base address and related index in an
array. Then, pinctrl driver configs its open-drain mode by
finding the corresponding 'channel' index.

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