serial: fsl_linflexuart: add support to change baudrate

commit 094ea122ff58f7ff26f3d38ec9b4d14c05442cff from
https://github.com/nxp-auto-linux/linux



Based on commit 8f788f3b2d90 ("tty: serial: linflexuart: Add initial
support for Treerunner Palladium")

Issue: ALB-9101

Signed-off-by: Adrian.Nitu <adrian.nitu@freescale.com>
Signed-off-by: Stefan-Gabriel Mirea <stefan-gabriel.mirea@nxp.com>
Signed-off-by: Radu Pirea <radu-nicolae.pirea@nxp.com>
Signed-off-by: Zhantao Tang <zhantao.tang@windriver.com>