ARM: EDMA: Fix clearing of unused list for DT DMA resources

HWMOD removal for MMC is breaking edma_start as the events are being manually
triggered due to unused channel list not being clear, Thanks to Balaji TK for
finding this issue.

This patch fixes the issue, by reading the "dmas" property from the DT node if
it exists and clearing the bits in the unused channel list.

Cc: Balaji T K <balajitk@ti.com>
Cc: Pantel Antoniou <panto@antoniou-consulting.com>
Signed-off-by: Joel Fernandes <joelf@ti.com>