dt-bindings: mtd: spi-nor: Allow two CS per device

dt-bindings: mtd: spi-nor: Allow two CS per device

The Xilinx QSPI controller has two advanced modes which allow the
controller to behave differently and consider two flashes as one single
storage.

One of these two modes is quite complex to support from a binding point
of view and is the dual parallel memories. In this mode, each byte of
data is stored in both devices: the even bits in one, the odd bits in
the other. The split is automatically handled by the QSPI controller and
is transparent for the user.

The other mode is simpler to support, it is called dual stacked
memories. The controller shares the same SPI bus but each of the devices
contain half of the data. Once in this mode, the controller does not
follow CS requests but instead internally wires the two CS levels with
the value of the most significant address bit.

Supporting these two modes will involve core changes which include the
possibility of providing two CS for a single SPI device

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20220126112608.955728-2-miquel.raynal@bootlin.com
Signed-off-by: Mark Brown <broonie@kernel.org>