dmaengine: rcar-dmac: Add support for R-Car S4-8

Add support for R-Car S4-8. We can reuse R-Car V3U code so that
renames variable names as "gen4".

Note that some registers of R-Car V3U do not exist on R-Car S4-8,
but none of them are used by the driver for now.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20211222114507.1252947-3-yoshihiro.shimoda.uh@renesas.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>