spi: dt-bindings: Add hold/inactive CS delay peripheral properties

These two properties complete the bindings for the Linux spi_device cs
model, which includes cs_setup, cs_hold and cs_inactive delay values.

Signed-off-by: Janne Grunau <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Hector Martin <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/spi/spi-peripheral-props.yaml

···
       Delay in nanoseconds to be introduced by the controller after CS is
       asserted.
 
+  spi-cs-hold-delay-ns:
+    description:
+      Delay in nanoseconds to be introduced by the controller before CS is
+      de-asserted.
+
+  spi-cs-inactive-delay-ns:
+    description:
+      Delay in nanoseconds to be introduced by the controller after CS is
+      de-asserted.
+
   spi-rx-bus-width:
     description:
       Bus width to the SPI bus used for read transfers.