spi: fsi: fsi2spi: Add compatible string for restricted version

Add a compatible string for the restricted version of the SPI controller.
The restricted version cannot process sequence loop operations and
therefore has a smaller transfer size.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Link: https://lore.kernel.org/r/20200909222857.28653-5-eajames@linux.ibm.com
Signed-off-by: Mark Brown <broonie@kernel.org>