[nrf fromtree] samples: drivers: counter: add nRF54L20 config

Added build configuration for nRF54L20.

Signed-off-by: Rafał Kuźnia <rafal.kuznia@nordicsemi.no>
(cherry picked from commit bcc7562)

tests/drivers/counter/counter_basic_api/boards/nrf54l20pdk_nrf54l20_cpuapp.overlay

@@ -0,0 +1,34 @@
+&timer00 {
+	prescaler = <6>;
+	status = "okay";
+};
+
+&timer10 {
+	prescaler = <4>;
+	status = "okay";
+};
+
+&timer20 {
+	prescaler = <4>;
+	status = "okay";
+};
+
+&timer21 {
+	prescaler = <4>;
+	status = "okay";
+};
+
+&timer22 {
+	prescaler = <4>;
+	status = "okay";
+};
+
+&timer23 {
+	prescaler = <4>;
+	status = "okay";
+};
+
+&timer24 {
+	prescaler = <4>;
+	status = "okay";
+};