uart_mcux_flexcomm: Fix TX garbage char bug in PM

There is currently a bug where when entering low power modes, there is a
garbage character sent if the UART is in the middle of a transmit when
entering low power mode. This is because the fifo interrupt happens when
last character is pulled by transmitter from fifo and then we unlock PM
constraints at that moment, instead of waiting for actual transmitter to
become idle. Fix by adding work item to check for this when needed.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>