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>