ARM: imx6: Rename DEBUG_IMX6Q_UART to UART4

Different boards may use differenct UART ports for debugging.
To be able to add different debug UART configurations (e.g. UART2),
rename the existing general DEBUG_IMX6Q_UART to DEBUG_IMX6Q_UART4.

Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
index e0d236d..87abb5d 100644
--- a/arch/arm/Kconfig.debug
+++ b/arch/arm/Kconfig.debug
@@ -199,12 +199,12 @@
 		  Say Y here if you want kernel low-level debugging support
 		  on i.MX50 or i.MX53.
 
-	config DEBUG_IMX6Q_UART
-		bool "i.MX6Q Debug UART"
+	config DEBUG_IMX6Q_UART4
+		bool "i.MX6Q Debug UART4"
 		depends on SOC_IMX6Q
 		help
 		  Say Y here if you want kernel low-level debugging support
-		  on i.MX6Q.
+		  on i.MX6Q UART4.
 
 	config DEBUG_S3C_UART0
 		depends on PLAT_SAMSUNG