driver: uart_npcx: Fix uart to use PM constrain

The device PM callback needs to be used only to suspend/resume devices.
If the system cannot be suspended because UART is in a particular
state, the pm_constraint_set/release API should be used. For NPCX UART,
the chip can't enter low power idle state until UART completes the data
transmission.
This commit changes NPCX UART to use pm_constraint_set/release & fixes
UART lost data from low power idle.

Fix #40621

Signed-off-by: Wealian Liao <WHLIAO@nuvoton.com>
Signed-off-by: Jun Lin <CHLin56@nuvoton.com>