serial: lpuart: Move CONFIG_FSL_LPUART to Kconfig

LPUART is seen on Freescale VF610 and QorIQ Layerscape devices.
Create a Kconfig option and move it to defconfig for all boards
that have this serial driver.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Stefan Agner <stefan.agner@toradex.com>
Acked-by: Simon Glass <sjg@chromium.org>