Unify sunxi SPI driver a little more

Currently the unification happened by merging spi_sun4i and spi_sun5i
but the KConfig's remained split. This merges the KConfig's to SPI_SUNXI
and adjusts the spi driver likewise. Integration with spi for sun7i
should be easier now.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>