sf: Provide a command to access memory-mapped SPI

Add a new 'sf mmap' function to show the address of a SPI offset, if the
hardware supports it. This is useful on x86 systems.

Cover-letter:
sf: Add documentation and an 'sf mmap' command
This little series adds documentation and a few other tidy-ups to the
'sf' command.

It also provides a way to access memory-mapped SPI via the command line.
END
Series-changes: 3
- Add configuration and return value also

Series-links: 256158 2:256134 1:237539
Series-version: 4
Series-to: u-boot
Series-cc: heinrich
Series-cc: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
40 jobs for sf-working in 45 minutes and 17 seconds (queued for 8 seconds)
latest
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