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/drivers/serial/Kconfig b/drivers/serial/Kconfig
index 92084a2..83068cf 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -228,6 +228,12 @@
 	  Select this to enable an UART for Altera devices. Please find
 	  details on the "Embedded Peripherals IP User Guide" of Altera.
 
+config FSL_LPUART
+	bool "Freescale LPUART support"
+	help
+	  Select this to enable a Low Power UART for Freescale VF610 and
+	  QorIQ Layerscape devices.
+
 config SYS_NS16550
 	bool "NS16550 UART or compatible"
 	help