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>

board/sandbox/sandbox/README.sandbox

@@ -31,6 +31,60 @@ the console. It does not set the terminal into raw mode, so cursor keys and
 history will not work yet.
 
 
+SPI Emulation
+-------------
+
+Sandbox supports SPI and SPI flash emulation.
+
+This is controlled by the spi_sf argument, the format of which is:
+
+   bus:cs:device:file
+
+   bus    - SPI bus number
+   cs     - SPI chip select number
+   device - SPI device emulation name
+   file   - File on disk containing the data
+
+For example:
+
+ dd if=/dev/zero of=spi.bin bs=1M count=4
+ ./u-boot --spi_sf 0:0:M25P16:spi.bin
+
+With this setup you can issue SPI flash commands as normal:
+
+=>sf probe
+SF: Detected M25P16 with page size 64 KiB, total 2 MiB
+=>sf read 0 0 10000
+SF: 65536 bytes @ 0x0 Read: OK
+=>
+
+Since this is a full SPI emulation (rather than just flash), you can
+also use low-level SPI commands:
+
+=>sspi 0:0 32 9f
+FF202015
+
+This is issuing a READ_ID command and getting back 20 (ST Micro) part
+0x2015 (the M25P16).
+
+Drivers are connected to a particular bus/cs using sandbox's state
+structure (see the 'spi' member). A set of operations must be provided
+for each driver.
+
+
+Configuration settings for the curious are:
+
+CONFIG_SANDBOX_SPI_MAX_BUS
+	The maximum number of SPI buses supported by the driver (default 1).
+
+CONFIG_SANDBOX_SPI_MAX_CS
+	The maximum number of chip selects supported by the driver
+	(default 10).
+
+CONFIG_SPI_IDLE_VAL
+	The idle value on the SPI bus
+
+
 Tests
 -----