ARM: dts: am43xx: add emif interrupt info

The EMIF instance on am43xx supports interrupts, so add the info for
completeness sake. Right now, the driver does not use this for anything,
but it is possible it would be used in future.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>