spi: cadence-qspi: assert each subnode flash CS is valid

spi: cadence-qspi: assert each subnode flash CS is valid

Check each flash CS against the num-cs property from devicetree.
Fallback to the driver max supported value (CQSPI_MAX_CHIPSELECT) if
num-cs isn't present.

cqspi->num_chipselect is set in cqspi_of_get_pdata() to the num-cs
devicetree property, or to CQSPI_MAX_CHIPSELECT if num-cs is not set.

Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
Reviewed-by: Dhruva Gole <d-gole@ti.com>
Link: https://msgid.link/r/20240209-cdns-qspi-cs-v1-1-a4f9dfed9ab4@bootlin.com
Signed-off-by: Mark Brown <broonie@kernel.org>