DMA: shdma: Fix warnings due to declared but unused symbols

Several functions and variables are use on SH_CPU4 or ARM only. Guard
their declaration with conditional compilation directives to avoid
warnings.

Cc: Guennadi Liakhovetski <g.liakhovetski+renesas@gmail.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Cc: dmaengine@vger.kernel.org
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>