dt-bindings: mtd: gpmc-onenand: Update properties description

Compatible property is required for OMAP2+ mtd driver. Also
add INT pin gpio description and delete unused dma-channel
property.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>