spi: docbook: add missing parameter documentation

Add missing docbook documentation for the gfp parameter
in function spi_replace_transfers.

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -2142,6 +2142,7 @@ static void __spi_replace_transfers_release(struct spi_master *master,
  * @release:       extra release code necessary in some circumstances
  * @extradatasize: extra data to allocate (with alignment guarantees
  *                 of struct @spi_transfer)
+ * @gfp:           gfp flags
  *
  * Returns: pointer to @spi_replaced_transfers,
  *          PTR_ERR(...) in case of errors.