ARM: imx: add dependency check for DEBUG_IMX_UART_PORT

While adding i.MX DEBUG_LL selection, commit f8c95fe (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: Karl Beldan <karl.beldan@gmail.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>