SPI: Split writes using spi_crop_chunk()

SPI controllers in Intel and AMD bridges have a slightly different
restriction on how long transactions they can handle.

Change-Id: I3d149d4b7e7e9633482a153d5e380a86c553d871
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/6163
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>

src/include/spi-generic.h

@@ -168,6 +168,8 @@ void spi_cs_deactivate(struct spi_slave *slave);
  */
 void spi_set_speed(struct spi_slave *slave, uint32_t hz);
 
+unsigned int spi_crop_chunk(unsigned int cmd_len, unsigned int buf_len);
+
 /*-----------------------------------------------------------------------
  * Write 8 bits, then read 8 bits.
  *   slave:	The SPI slave we're communicating with