ARM: imx: add dependency check for DEBUG_IMX_UART_PORT

While adding i.MX DEBUG_LL selection, commit f8c95fe6 (ARM: imx: support
DEBUG_LL uart port selection for all i.MX SoCs) leaves Kconfig symbol
DEBUG_IMX_UART_PORT there without any dependency check.  This results in
that everyone gets the symbol in their config, which is someting
undesirable.  Add "depends on ARCH_MXC" for the symbol to prevent that.
Reported-by: NKarl Beldan <karl.beldan@gmail.com>
Signed-off-by: NShawn Guo <shawn.guo@linaro.org>

arch/arm/Kconfig.debug

@@ -495,6 +495,7 @@ config DEBUG_IMX_UART_PORT
 						DEBUG_IMX53_UART || \
 						DEBUG_IMX6Q_UART
 	default 1
+	depends on ARCH_MXC
 	help
 	  Choose UART port on which kernel low-level debug messages
 	  should be output.