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>
diff --git a/configs/ls1021aqds_nor_lpuart_defconfig b/configs/ls1021aqds_nor_lpuart_defconfig
index b2f6832..1186af2 100644
--- a/configs/ls1021aqds_nor_lpuart_defconfig
+++ b/configs/ls1021aqds_nor_lpuart_defconfig
@@ -4,3 +4,4 @@
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
+CONFIG_FSL_LPUART=y