dt: spi: Add Renesas RZ/N1 binding documentation

The Renesas RZ/N1 SPI Controller is based on the Synopsys DW SSI, but has
additional registers for software CS control and DMA. This patch does not
address the changes required for DMA support, it simply adds the compatible
string. The CS functionality is not very useful and also not needed as
Linux can use gpios for the CS signals.

Add a compatible string to handle any unforeseen issues that may arise, and
pave the way for DMA support.
Signed-off-by: Gareth Williams <gareth.williams.jx@renesas.com>
Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Link: https://lore.kernel.org/r/1568793876-9009-2-git-send-email-gareth.williams.jx@renesas.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/renesas,rzn1-spi.txt

+Renesas RZ/N1 SPI Controller
+
+This controller is based on the Synopsys DW Synchronous Serial Interface and
+inherits all properties defined in snps,dw-apb-ssi.txt except for the
+compatible property.
+
+Required properties:
+- compatible : The device specific string followed by the generic RZ/N1 string.
+   Therefore it must be one of:
+   "renesas,r9a06g032-spi", "renesas,rzn1-spi"
+   "renesas,r9a06g033-spi", "renesas,rzn1-spi"