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>