boards: silabs: siwx91x: Add support for PSRAM

Radio Board 4342A has 8MB of PSRAM connected to the Quad-SPI memory
controller.

The memory controller of the 4342A is preconfigured by the bootloader.
So, the PSRAM work out of the box without any manual configuration.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>