serial: fsl_lpuart: add loopback support

The LPUART can loop the RX and TX signal. Add support for it.

Please note, this was only tested on the 32 bit version of the LPUART.

Signed-off-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20210512141255.18277-9-michael@walle.cc


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>