dmaengine: sh: Remove R-Mobile APE6 support

Renesas R-Mobile APE6 support is currently unused:
  - DMA slaves were never enabled in r8a73a4.dtsi,
  - The driver relies on legacy filter matching and describing all
    slaves and MID/RIDs in a table, unlike modern DMA engine drivers for
    similar hardware like rcar-dmac,
  - The driver doesn't seem to work well.

Remove the driver, it can be resurrected from git history when needed.

As this was the last user of SH_DMAE_BASE on Renesas ARM SoCs, the
sh-dma-engine driver core is now used on SuperH only.

Note that the DT bindings are still present, as r8a73a4.dtsi uses them.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Vinod Koul <vkoul@kernel.org>