spi: dt-bindings: support devices with multiple chipselects

Add binding support for devices, that have more than one
chip select. A typical example are SPI connected microcontroller,
that can also be programmed over SPI like NXP Kinetis or
chips with a configuration and a data chip select, such as
Microchip's MRF89XA transceiver.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://lore.kernel.org/r/20210621175359.126729-3-sebastian.reichel@collabora.com


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