ARM: edma: Add EDMA crossbar event mux support

EDMA supports a cross bar which provides ability
to mux additional events into physical channels
present in the channel controller.

This is required when the number of events present
in the system are more than number of available
physical channels.

Changes by Joel:
* Split EDMA xbar support out of original EDMA DT parsing patch
to keep it easier for review.
* Rewrite shift and offset calculation.

Suggested-by: Sekhar Nori <nsekhar@ti.com>
Suggested by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Joel A Fernandes <joelagnel@ti.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
[nsekhar@ti.com: fix checkpatch errors and a minor coding improvement]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>