dmaengine: Kconfig: Update description for RCAR_DMAC config

dmaengine: Kconfig: Update description for RCAR_DMAC config

rcar-dmac driver is used on Renesas R-Car Gen{2,3} and Renesas
RZ/G{1,2} SoC's, update the same to reflect the description
for RCAR_DMAC config.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Chris Paterson <Chris.Paterson2@renesas.com>
Link: https://lore.kernel.org/r/20200911095734.19348-1-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>