boards: silabs: siwx917_rb4342a: Add SPI to supported features

Add SPI to the supported list in siwx917_rb4342a.yaml to reflect
hardware capability on the PSRAM-based SiWx917 Radio Board.

Signed-off-by: Sai Santhosh Malae <[email protected]>

Author: smalae

boards/silabs/radio_boards/siwx917_rb4342a/siwx917_rb4342a.yaml

@@ -21,5 +21,6 @@ supported:
   - uart
   - watchdog
   - counter
+  - spi
   - wifi
 vendor: silabs