board: sensry: ganymed-sk - fix missing gpio/NET_L2

when compiling net-samples then the required gpio is disabled.
the installed vsc8541 phy requires gpio for reset. additionally,
we enable NET_L2 when network is enabled.

Signed-off-by: Sven Ginka <s.ginka@sensry.de>

Author: tswaehn

boards/sensry/ganymed_sk/Kconfig.defconfig

@@ -0,0 +1,5 @@
+# Copyright (c) 2025 sensry.io
+# SPDX-License-Identifier: Apache-2.0
+
+configdefault NET_L2_ETHERNET
+	default y