dma: ste_dma40: Expand DT binding to accept 'high-priority channel' flag

This is used for MSP (audio) devices which is about to be fully DT:ed.

Cc: devicetree@vger.kernel.org
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>