spi: dt-bindings: Document CS active-high

The current documentation does not clearly explain how to invert the SPI
CS signal to make it active-high. This makes it very difficult to
understand.

This patch adds a simple explanation on how to set the CS line in
active-high and adds an example to make it easier for users who need
that setup for their SPI peripherals.

Link: https://forums.raspberrypi.com/viewtopic.php?t=378222


Signed-off-by: Iker Pedrosa <ikerpedrosam@gmail.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20241216095739.27320-1-ikerpedrosam@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>