dma: mv_xor: add Device Tree binding

This patch finally adds a Device Tree binding to the mv_xor
driver. Thanks to the previous cleanup patches, the Device Tree
binding is relatively simply: one DT node per XOR engine, with
sub-nodes for each XOR channel of the XOR engine. The binding
obviously comes with the necessary documentation.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: devicetree-discuss@lists.ozlabs.org