ARM: imx: fix typo "DEBUG_IMX50_IMX53_UART"

Commit f8c95fe6 (ARM: imx: support DEBUG_LL uart port selection for all
i.MX SoCs) had a typo that DEBUG_IMX50_IMX53_UART should be
DEBUG_IMX53_UART.
Signed-off-by: NPaul Bolle <pebolle@tiscali.nl>
Signed-off-by: NShawn Guo <shawn.guo@linaro.org>

arch/arm/Kconfig.debug

@@ -492,7 +492,7 @@ config DEBUG_IMX_UART_PORT
 						DEBUG_IMX31_UART || \
 						DEBUG_IMX35_UART || \
 						DEBUG_IMX51_UART || \
-						DEBUG_IMX50_IMX53_UART || \
+						DEBUG_IMX53_UART || \
 						DEBUG_IMX6Q_UART
 	default 1
 	help