sample: net: fix the board config and overlay name

Rename the board config to fix ci issue while running samples for
fvp_baser_aemv8r:
```
INFO    - 1) sample.net.zperf.async_tx.stm32 on
fvp_baser_aemv8r/fvp_aemv8r_aarch32 error (CMake build failure - CMake
Error at cmake/modules/extensions.cmake:2905 (message):   Board
fvp_baser_aemv8r defines multiple SoCs.)
```

Signed-off-by: Sudan Landge <[email protected]>

Author: wearyzen

samples/net/zperf/boards/fvp_baser_aemv8r.overlay

@@ -0,0 +1,17 @@
+/*
+ * Copyright 2025 Arm Limited and/or its affiliates <[email protected]>
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+&eth {
+	status = "okay";
+};
+
+&phy {
+	status = "okay";
+};
+
+&mdio {
+	status = "okay";
+};