ARM: imx: add low-level debug support for i.mx6ul

Enable low-level debug support for i.MX6UL by adding the
debug port definitions for the SoC.

Singed-off-by: Anson Huang <b20788@freescale.com>
Signed-off-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>