uboot-phytec

spi: Add SPI flash test

It is useful to have a basic SPI flash test, which tests that the SPI chip,
the SPI bus and the driver are behaving.

This test erases part of the flash, writes data and reads it back as a
sanity check that all is well.

Use CONFIG_SF_TEST to enable it.

Signed-off-by: Simon Glass <sjg@chromium.org>