fix: drop s3, c3 support for pppos test apps

no SOC_UART_SUPPORT_REF_TICK for these targets

tools/test_apps/protocols/pppos/README.md

@@ -1,3 +1,6 @@
+| Supported Targets | ESP32 | ESP32-S2 |
+| ----------------- | ----- | -------- |
+
 # PPP over Serial test
 
 Testing connection of PPP client (ESP) to PPP server (raspberry pi)