ARM: dts: add generic DMA device tree binding for mxs-dma

Add generic DMA device tree binding for mxs-dma.  The changes include:

 * Add channel interrupts into DMA controller nodes
 * Add properties '#dma-cells' and 'dma-channels' for DMA controller nodes
 * And properties 'dmas' and 'dma-names' for DMA client nodes
 * Update mxs-dma device tree binding doc

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