spi: dw: document Microsemi integration

The integration of the Designware SPI controller on Microsemi SoCs requires
an extra register set to be able to give the IP control of the SPI
interface.

Cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>