drivers/serial: uart_ns16550: add power management constraint API

Instead of busy wait until completed transaction, the constraint set
is used before enabling tx interrupt to not allow system to enter
suspend when tx is transmitting.

Application defined policy should use the pm_constraint_get function
to check if given state is enabled and could be used.

TEST=Test on hayato board(soc:it8xxx2), the logs print normally before
system enters suspend.

Signed-off-by: Tim Lin <tim2.lin@ite.corp-partner.google.com>