dmaengine: sh: Rework Kconfig and Makefile

Separate helpers and drivers in the Kconfig and Makefile to improve
readability and move the CONFIG_OF dependency from the Makefile to
Kconfig.

[pebolle@tiscali.nl: reported need to rename SHDMA_R8A73A4 instances]
Reported-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
[horms+renesas@verge.net.au: squashed rename of SHDMA_R8A73A4 instances]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>