spi: cadence_xspi: enable automatic platform data allocation

spi: cadence_xspi: enable automatic platform data allocation

Enable automatic allocation of platform data for the Cadence XSPI
controller by setting .plat_auto.

Without this, dev_get_plat() may return invalid or uninitialized
platform data when multiple XSPI controllers are present, leading
to incorrect IOBASE/SDMABASE/AUXBASE values and causing SPI flash
probe failures.

Setting .plat_auto ensures each controller instance receives a
properly sized cdns_xspi_plat structure, allowing SF probe to work
correctly.

Tested on an Altera Simics platform with multiple XSPI controllers.

Signed-off-by: Chen Huei Lok <chen.huei.lok@altera.com>