tty: serial: fsl_lpuart: Use scatter/gather DMA for Tx

Drop PIO to DMA switching and use scatter/gather DMA for Tx path to improve
performance.

Some part of the code is borrowed from imx serial driver.
Signed-off-by: Bhuvanchandra DV <bhuvanchandra.dv@toradex.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>