ARM: imx6: Add UART2 for low-level debug

To be able to enable early debugging on boards using the UART2 for the
console, add the option for early debugging on UART2.

Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>