driver: serial: npcx: add PM support for npck3 series.

This commit adds power management support to the UART driver for npck3
series chip.

Signed-off-by: Alvis Sun <yfsun@nuvoton.com>
Signed-off-by: Tom Chang <CHChang19@nuvoton.com>
Signed-off-by: Mulin Chao <mlchao@nuvoton.com>