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>

Documentation/devicetree/bindings/spi/snps,dw-apb-ssi.txt

 Synopsys DesignWare AMBA 2.0 Synchronous Serial Interface.
 
 Required properties:
-- compatible : "snps,dw-apb-ssi"
-- reg : The register base for the controller.
+- compatible : "snps,dw-apb-ssi" or "mscc,<soc>-spi", where soc is "ocelot" or
+  "jaguar2"
+- reg : The register base for the controller. For "mscc,<soc>-spi", a second
+  register set is required (named ICPU_CFG:SPI_MST)
 - interrupts : One interrupt, used by the controller.
 - #address-cells : <1>, as required by generic SPI binding.
 - #size-cells : <0>, also as required by generic SPI binding.