dma: add dma_get_any_slave_channel(), for use in of_xlate()

dma: add dma_get_any_slave_channel(), for use in of_xlate()

mmp_pdma.c implements a custom of_xlate() function that is 95% identical
to what Tegra will need. Create a function to implement the common part,
so everyone doesn't just cut/paste the implementation.

Cc: Dan Williams <[email protected]>
Cc: Vinod Koul <[email protected]>
Cc: Lars-Peter Clausen <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Stephen Warren <[email protected]>
Signed-off-by: Vinod Koul <[email protected]>