dt-bindings: spi: Document Ingenic SPI controller bindings

Add a documentation file to describe the Device Tree bindings for the
SPI controller found in Ingenic SoCs.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Artur Rojek <contact@artur-rojek.eu>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210830230139.21476-2-contact@artur-rojek.eu


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