ARM: dts: imx: Mark IIM as syscon on i.MX51/i.MX53

IIM contains system fuses with information like SoC unique ID
(serial) on i.MX51 and i.MX53. Add "syscon" compatible allowing
simple access.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>