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/vf610twr_nand_defconfig b/configs/vf610twr_nand_defconfig
index 98880f3..299fa8f 100644
--- a/configs/vf610twr_nand_defconfig
+++ b/configs/vf610twr_nand_defconfig
@@ -6,3 +6,4 @@
 CONFIG_NAND_VF610_NFC=y
 CONFIG_SYS_NAND_BUSWIDTH_16BIT=y
 CONFIG_SPI_FLASH=y
+CONFIG_FSL_LPUART=y