spi: dt-bindings: Add num-cs property for mpfs-spi

spi: dt-bindings: Add num-cs property for mpfs-spi

The PolarFire SoC SPI "hard" controller supports eight CS lines, out of
which only one CS line is physically wired. The default value of
'num-cs' was never set and it did not didn't impose a maximum value.

To reflect this hardware limitation in the device tree, the binding
enforces that the 'num-cs' property cannot exceed 1 unless additional
CS lines are explicitly defined using GPIO descriptors.

Fixes: 2da187304e55 ("spi: add bindings for microchip mpfs spi")
Signed-off-by: Prajna Rajendra Kumar <prajna.rajendrakumar@microchip.com>
Link: https://msgid.link/r/20240514104508.938448-2-prajna.rajendrakumar@microchip.com
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Mark Brown <broonie@kernel.org>