dmaengine: idma64: Move driver name to the header

There are two drivers that are relying on the iDMA 64-bit driver name
to match. Instead of duplicating string in both of them, dedicate
a header file and share it between users.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>