ARM: imx: add debug uart support for IMX50 SoC

Add appropriate UART address definitions and support defines for using the
UARTs of the Freescale IMX50 SoC as debug ports.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>