driver: uart: npcx: Add pm_deivce support for uart driver.

Since the uart device clock will be stopped after ec entered sleep or
deep sleep state and restore its clock automatically, there is no need
to implement code for suspending and resuming devices manually.

The driver still needs to check the current status of uart device when
it wants to change its power state to LOW or SUSPEND power state. It is
crucial to forbid ec enters sleep or deep sleep state when uart device
is busy with transmitting data. Or we will observe broken characters on
the uart console.

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