ARM: dts: am437x: Update GPMC dma channel to reflect new edma binding

Commit bbbe6750
("ARM: DTS: am33xx: Use the new DT bindings for the eDMA3") which updated
various dts to use the new DT bindings for the eDMA forgot to make the
change for am437x gpmc node.

This patch fixes this oversight to get NAND with dma prefetch working
again.

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>