dma: ste_dma40: Parse flags property for new 'high priority channel' request

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

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>