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/Makefile b/drivers/mtd/spi/Makefile
index 1bbeb7d..26483a2 100644
--- a/drivers/mtd/spi/Makefile
+++ b/drivers/mtd/spi/Makefile
@@ -13,4 +13,5 @@
 obj-$(CONFIG_CMD_SF)        += sf.o
 obj-$(CONFIG_SPI_FLASH) += sf_probe.o sf_ops.o
 obj-$(CONFIG_SPI_FRAM_RAMTRON) += ramtron.o
+obj-$(CONFIG_SPI_FLASH_SANDBOX) += sandbox.o
 obj-$(CONFIG_SPI_M95XXX) += eeprom_m95xxx.o