sandbox: spi: Add new SPI flash driver

This adds a SPI flash driver which simulates SPI flash clients.
Currently supports the bare min that U-Boot requires: you can
probe, read, erase, and write.  Should be easy to extend to make
it behave more exactly like a real SPI flash, but this is good
enough to merge now.

sjg@chromium.org added a README and tidied up code a little.
Added a required map_sysmem() for sandbox.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/mtd/spi/sf_probe.c b/drivers/mtd/spi/sf_probe.c
index ca12715..c1eb754 100644
--- a/drivers/mtd/spi/sf_probe.c
+++ b/drivers/mtd/spi/sf_probe.c
@@ -13,6 +13,7 @@
 #include <malloc.h>
 #include <spi.h>
 #include <spi_flash.h>
+#include <asm/io.h>
 
 #include "sf_internal.h"
 
@@ -279,7 +280,7 @@
 		debug("%s: Memory map must cover entire device\n", __func__);
 		return -1;
 	}
-	flash->memory_map = (void *)addr;
+	flash->memory_map = map_sysmem(addr, size);
 
 	return 0;
 }