ARM: debug-imx: only define DEBUG_IMX_UART_PORT if needed

ARM: debug-imx: only define DEBUG_IMX_UART_PORT if needed

If debugging on i.MX is enabled DEBUG_IMX_UART_PORT defines which UART
is used for the debug output. If however debugging is off don't only
hide the then unused config item but drop it completely by using a
dependency instead of a conditional prompt.

This fixes DEBUG_IMX_UART_PORT being present in the kernel config even
if DEBUG_LL is disabled.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>