fix a few RISC-V / renesas Kconfig dependencies

[Christoph Hellwig]

Hi all,

this small series sorts out a few Kconfig dependency issue for the
rich set of RISC-V non-coherent DMA support.

Geert suggested for it to go into the soc tree, which is fine with me
if I get a stable branch for it as it is required for further dma-mapping
work.