boards: silabs: siwx91x: Fix compatibility with BTP

The serial pipe driver is required when the Zephyr device acts as the
remote of a host application. Especially, frameworks like BTP rely on
this driver.

Until now, the "zephyr,uart-pipe" attribute was missing. So the serial
pipe driver was unable to start.

Signed-off-by: Jérôme Pouiller <[email protected]>

Author: jerome-pouiller

boards/silabs/radio_boards/siwx917_rb4338a/siwx917_rb4338a.dts

@@ -20,6 +20,7 @@
 		zephyr,code-partition = &code_partition;
 		zephyr,console = &ulpuart;
 		zephyr,shell-uart = &ulpuart;
+		zephyr,uart-pipe = &ulpuart;
 		zephyr,bt-hci = &bt_hci0;
 	};