spi: dt-bindings: cdns,qspi-nor: make cdns,fifo-depth optional

Make cdns,fifo-depth devicetree property optional.
Value can be detected at runtime.

Upper SRAMPARTITION register bits are read-only. Procedure to find FIFO
depth is therefore to write 0xFFFFFFFF and read back to get amount of
writeable bits.
Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
Link: https://msgid.link/r/20240410-cdns-qspi-mbly-v3-3-7b7053449cf7@bootlin.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/cdns,qspi-nor.yaml

@@ -146,7 +146,6 @@ required:
   - reg
   - interrupts
   - clocks
-  - cdns,fifo-depth
   - cdns,fifo-width
   - cdns,trigger-address
   - '#address-cells'