dmaengine: sh: rcar-dmac: add a new function to clear CHCR.DE with barrier

This patch adds a new function rcar_dmac_clear_chcr_de() to simplify
adding pause function later.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>