ARM: imx6q: let users input debug uart port number

imx6q gets 5 uart ports in total.  Different board design may choose
different port as debug uart.  For example, imx6q-sabresd uses UART1,
imx6q-sabrelite uses UART2 and imx6q-arm2 uses UART4.  Rather than
bloating DEBUG_LL choice list with all these uart ports, the patch
introduces DEBUG_IMX6Q_UART_PORT for users to input uart port number
when DEBUG_IMX6Q_UART is selected inside DEBUG_LL choice.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>