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 <jerome.pouiller@silabs.com>
1 file changed