ARM i.MX31: Add devicetree support

This patch adds basic devicetree support for i.MX31 based SoCs.

Only the UART and interrupts bindings are added.

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@no-log.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>