dm: sandbox: Add a SPI emulation uclass

U-Boot includes a SPI emulation driver already but it is not explicit, and
is hidden in the SPI flash code.

Conceptually with sandbox's SPI implementation we have a layer which
creates SPI bus transitions and a layer which interprets them, currently
only for SPI flash. The latter is actually an emulation, and it should be
possible to add more than one emulation - not just SPI flash.

Add a SPI emulation uclass so that other emulations can be plugged in to
support different types of emulated devices on difference buses/chip
selects.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>

drivers/spi/Makefile

@@ -8,6 +8,7 @@
 # There are many options which enable SPI, so make this library available
 ifdef CONFIG_DM_SPI
 obj-y += spi-uclass.o
+obj-$(CONFIG_SANDBOX) += spi-emul-uclass.o
 else
 obj-y += spi.o
 endif

drivers/spi/spi-emul-uclass.c

@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2014 Google, Inc
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+#include <common.h>
+#include <dm.h>
+#include <spi.h>
+#include <spi_flash.h>
+
+UCLASS_DRIVER(spi_emul) = {
+	.id		= UCLASS_SPI_EMUL,
+	.name		= "spi_emul",
+};