Add documentation for the FSI2SPI CFAM engine, which provides access to
a number of SPI controllers.
Signed-off-by: Eddie James <eajames@linux.ibm.com>
Link: https://lore.kernel.org/r/20200306194118.18581-2-eajames@linux.ibm.comSigned-off-by: Mark Brown <broonie@kernel.org>