samples: zperf: use systick timer for RT1050

Use systick timer for RT1050 to provide better performance than low
frequency GPT timer

Performance change (as tested with iperf on RT1050)
TCP RX: 49.1Mbps->50.8Mbps
TCP TX: 40.8Mbps->46.2Mbps

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>

samples/net/zperf/boards/mimxrt1050_evk.overlay

@@ -0,0 +1,14 @@
+/*
+ * Copyright 2022 NXP
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/* Use cortex systick as hardware timer */
+&systick {
+	status = "okay";
+};
+
+&gpt_hw_timer {
+	status = "disabled";
+};