ARM: dts: am437x: Fix GPMC dma properties

This patch updates the GPMC's DT DMA property to reflect the updated eDMA
bindings.

Fixes: cce1ee00 ("ARM: DTS: am437x: Use the new DT bindings for the eDMA3")

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Acked-by: Roger Quadros <rogerq@ti.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>