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>

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