dt-bindings: net: silabs,wfx: add prt,prtt1c-wfm200 antenna variant

Add compatible for wfm200 antenna configuration variant for Protonic PRTT1C
board.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>