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>