spi: Use void pointers for data in simple SPI I/O operations

Currently the simple SPI I/O operations all take pointers to u8 * buffers
to operate on. This creates needless type compatibility issues and the
underlying spi_transfer structure uses void pointers anyway so convert the
API over to take void pointers too.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>