soc: arm: nxp: switch imxrt boards to use systick timer unless CONFIG_PM=y

Switch all imxrt boards to use the systick timer by default, and only
enable the GPT timer when using low power modes. This is desirable
because the systick has a higher resolution, but the GPT can run
while the core clock is gated, making it useful for low power modes.

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