spi: Add parameter for maximum write size

Some SPI controllers (e.g. Intel ICH) have a limit on the number of SPI
bytes that can be written at a time. Add this as a parameter so that
clients of the SPI interface can respect this value.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/spi.h

@@ -49,10 +49,13 @@
  *
  *   bus:	ID of the bus that the slave is attached to.
  *   cs:	ID of the chip select connected to the slave.
+ *   max_write_size:	If non-zero, the maximum number of bytes which can
+ *		be written at once, excluding command bytes.
  */
 struct spi_slave {
 	unsigned int	bus;
 	unsigned int	cs;
+	unsigned int max_write_size;
 };
 
 /*-----------------------------------------------------------------------