When G2_DMA is enabled and SH_DMA is disabled, it results in the following
Kbuild warning:

WARNING: unmet direct dependencies detected for SH_DMA_API
  Depends on [n]: SH_DMA [=n]
  Selected by [y]:
  - G2_DMA [=y] && SH_DREAMCAST [=y]

The reason is that G2_DMA selects SH_DMA_API without depending on or
selecting SH_DMA while SH_DMA_API depends on SH_DMA.

When G2_DMA was first introduced with commit 40f49e7e
("sh: dma: Make G2 DMA configurable."), this wasn't an issue since
SH_DMA_API didn't have such dependency, and this way was the only way to
enable it since SH_DMA_API was non-visible. However, later SH_DMA_API was
made visible and dependent on SH_DMA with commit d8902adc
("dmaengine: sh: Add Support SuperH DMA Engine driver").

Let G2_DMA depend on SH_DMA_API instead to avoid Kbuild issues.

Fixes: d8902adc ("dmaengine: sh: Add Support SuperH DMA Engine driver")
Signed-off-by: Necip Fazil Yildiran <fazilyildiran@gmail.com>
Signed-off-by: Rich Felker <dalias@libc.org>