sandbox: update documents regarding spi_sf

Since the commit 1289e96797bf ("sandbox: spi: Drop command-line SPI
option"), "--spi_sf" command line option is no longer supported.

So update the following documents to sync them up with the change.
	doc/arch/sandbox.rst
	doc/SPI/README.sandbox-spi

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
2 files changed