Documentation: bindings: dma: Add binding for dma-channel-mask

Some dma channels can be reserved for secure mode or other
hardware on the SoC, so provide a binding for a bitmask
listing the available channels for the kernel to use.

This follows the pre-existing bcm,dma-channel-mask binding.

Cc: Vinod Koul <vkoul@kernel.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Tanglei Han <hantanglei@huawei.com>
Cc: Zhuangluan Su <suzhuangluan@hisilicon.com>
Cc: Ryan Grachek <ryan@edited.us>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Cc: dmaengine@vger.kernel.org
Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>