mmc: sandbox: Add support for writing

This adds support writing to the sandbox mmc backed by an in-memory
buffer. The unit test has been updated to test reading, writing, and
erasing. I'm not sure what MMCs erase to; I picked 0, but if it's 0xFF
then that can be easily changed.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>