dt-bindings: dmaengine: shdma: Rename bindings documentation file

Rename the bindings documentation file for shdma
from shdma.txt to renesas,shdma.txt.

This is part of an ongoing effort to name bindings documentation files for
Renesas IP blocks consistently, in line with the compat strings they
document.
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Link: https://lore.kernel.org/r/20190724114946.14021-1-horms+renesas@verge.net.au
[vkoul: add subsystem name]
Signed-off-by: Vinod Koul <vkoul@kernel.org>