dm: spi: Add a uclass for SPI

Add a uclass which provides access to SPI buses and includes operations
required by SPI.

For a time driver model will need to co-exist with the legacy SPI interface
so some parts of the header file are changed depending on which is in use.
The exports are adjusted also since some functions are not available with
driver model.

Boards must define CONFIG_DM_SPI to use driver model for SPI.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
(Discussed some follow-up comments which will address in future add-ons)

include/dm/uclass-id.h

@@ -22,6 +22,8 @@ enum uclass_id {
 	/* U-Boot uclasses start here */
 	UCLASS_GPIO,		/* Bank of general-purpose I/O pins */
 	UCLASS_SERIAL,		/* Serial UART */
+	UCLASS_SPI,		/* SPI bus */
+	UCLASS_SPI_GENERIC,	/* Generic SPI flash target */
 
 	UCLASS_COUNT,
 	UCLASS_INVALID = -1,