acpi: Support generation of SPI descriptor

Add a function to write a SPI descriptor to the generated ACPI code.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/include/spi.h b/include/spi.h
index a37900b..98ba9e7 100644
--- a/include/spi.h
+++ b/include/spi.h
@@ -13,8 +13,8 @@
 #include <linux/bitops.h>
 
 /* SPI mode flags */
-#define SPI_CPHA	BIT(0)			/* clock phase */
-#define SPI_CPOL	BIT(1)			/* clock polarity */
+#define SPI_CPHA	BIT(0)	/* clock phase (1 = SPI_CLOCK_PHASE_SECOND) */
+#define SPI_CPOL	BIT(1)	/* clock polarity (1 = SPI_POLARITY_HIGH) */
 #define SPI_MODE_0	(0|0)			/* (original MicroWire) */
 #define SPI_MODE_1	(0|SPI_CPHA)
 #define SPI_MODE_2	(SPI_CPOL|0)