spi: add support for bits-per-word setting

Allow dynamic configuration of the SPI word length. This is required
for controllers and slaves that need to operate with non-standard
word lengths, such as 9-bit wide transfers.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Simon Glass <simon.glass@canonical.com>

include/spi.h

@@ -77,11 +77,13 @@ struct dm_spi_bus {
  * @cs:		Chip select number (0..n-1)
  * @max_hz:	Maximum bus speed that this slave can tolerate
  * @mode:	SPI mode to use for this device (see SPI mode flags)
+ * @wordlen:	Word length in bits to use for this device
  */
 struct dm_spi_slave_plat {
 	unsigned int cs[SPI_CS_CNT_MAX];
 	uint max_hz;
 	uint mode;
+	unsigned int wordlen;
 };
 
 /**
@@ -718,6 +720,18 @@ int dm_spi_claim_bus(struct udevice *dev);
  */
 void dm_spi_release_bus(struct udevice *dev);
 
+/**
+ * Set the word length for SPI transactions
+ *
+ * Set the word length (number of bits per word) for SPI transactions.
+ *
+ * @slave:	The SPI slave
+ * @wordlen:	The number of bits in a word
+ *
+ * Returns: 0 on success, -1 on failure.
+ */
+int dm_spi_set_wordlen(struct udevice *dev, unsigned int wordlen);
+
 /**
  * SPI transfer
  *