drivers: serial: uart_mcux_lpuart: Fix pinctrl flow control

Currently the driver code does not actually request the flow control
state, it only checks if the state exists. To fix this we should
simply call pinctrl_apply_state() and if that fails, meaning no
flow control state exists, fall back to default pin mux settings.

Another case that needed fixing is run time flow control enabling
ie in the case where we have a WIFI/bluetooth module connected
and we initially operate without flow control but later enable
it after firmware download.

This patch addresses this as well by making a generic function
to achieve this.

Signed-off-by: Bas van Loon <bas@arch-embedded.com>