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.