samples: task_wdt: Increase min watchdog timeout

MCUX watchdog timer supports min window of 500ms. Increase maximum task
watchdog timeout window to 500ms by default, so that this sample can run
on iMX RT SOCs. Also update iMX RT watchdog driver to reject timeout
maximums under 500ms with a useful error message

Fixes #40153

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