dt-bindings: adis16460: Add 'spi-cs-inactive-delay-ns' property

The adis16460 device requires a stall time between SPI
transactions (during which the chip select is inactive),
with a minimum value equal to 16 microseconds.
This commit adds 'spi-cs-inactive-delay-ns' property, which should
indicate the stall time between consecutive SPI transactions.
The specified minimum time may not be sufficient for all
configurations.

Signed-off-by: Ramona Gradinariu <ramona.gradinariu@analog.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20231027140358.328699-4-ramona.gradinariu@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>