ARM: dts: imx: remove old DMA binding data from gpmi node

After mxs-dma driver adopts generic DMA device tree binding, gpmi
channel interrupt number is defined in DMA controller node, and
channel ID is listed in "dmas" property.  So the DMA channel interrupt
number in gpmi node "interrupts" property and fsl,gpmi-dma-channel which
are used by old customized DMA binding can be removed now.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>