dt-bindings: trivial-devices: Document CPLD on DH electronics boards

The CPLD on DH electronics boards is used to model arbitrary custom
glue logic, however it does have SPI interface. Document the CPLD DT
binding as trivial SPI device.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: devicetree@vger.kernel.org
Cc: NXP Linux Team <linux-imx@nxp.com>
To: linux-spi@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220407210106.256027-1-marex@denx.de