spi: Remove unused spi_init

Remove spi_init definition which never used on
respective code since from many years.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>

include/spi.h

@@ -117,13 +117,6 @@ struct spi_slave {
 #define SPI_XFER_MMAP_END	BIT(3)	/* Memory Mapped End */
 };
 
-/**
- * Initialization, must be called once on start up.
- *
- * TODO: I don't think we really need this.
- */
-void spi_init(void);
-
 /**
  * spi_do_alloc_slave - Allocate a new SPI slave (internal)
  *