spi: dt-bindings: introduce FIFO depth properties

There are SPI IPs that can be configured by the integrator with a
specific FIFO depth depending on the system's capabilities. For example,
the samsung USI SPI IP can be configured by the integrator with a TX/RX
FIFO from 8 byte to 256 bytes.

Introduce the ``fifo-depth`` property for such instances of IPs where the
same FIFO depth is used for both RX and TX. Introduce ``rx-fifo-depth``
and ``tx-fifo-depth`` properties for cases where the RX FIFO depth is
different from the TX FIFO depth.

Make the dedicated RX/TX properties dependent on each other and mutual
exclusive with the other.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://msgid.link/r/20240216070555.2483977-2-tudor.ambarus@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-controller.yaml

@@ -69,6 +69,21 @@ properties:
          Should be generally avoided and be replaced by
          spi-cs-high + ACTIVE_HIGH.
 
+  fifo-depth:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description:
+      Size of the RX and TX data FIFOs in bytes.
+
+  rx-fifo-depth:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description:
+      Size of the RX data FIFO in bytes.
+
+  tx-fifo-depth:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description:
+      Size of the TX data FIFO in bytes.
+
   num-cs:
     $ref: /schemas/types.yaml#/definitions/uint32
     description:
@@ -116,6 +131,10 @@ patternProperties:
       - compatible
       - reg
 
+dependencies:
+  rx-fifo-depth: [ tx-fifo-depth ]
+  tx-fifo-depth: [ rx-fifo-depth ]
+
 allOf:
   - if:
       not:
@@ -129,6 +148,14 @@ allOf:
       properties:
         "#address-cells":
           const: 0
+  - not:
+      required:
+        - fifo-depth
+        - rx-fifo-depth
+  - not:
+      required:
+        - fifo-depth
+        - tx-fifo-depth
 
 additionalProperties: true