tty: serial: fsl_lpuart: add earlycon for imx8qxp platform

Add earlycon support for imx8qxp platform.

Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Link: https://lore.kernel.org/r/20190717051930.15514-3-fugang.duan@nxp.com


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