dt-bindings: trivial-devices: Document CPLD on Menlo boards

The CPLD on Menlo boards is used to operate custom hardware,
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/20220407210131.256045-1-marex@denx.de