boards: arm: update documentation for imxrt to reflect use of systick

Update documentation for imxrt boards to reflect the default clock
source being systick, and clarify that the GPT timer will only be
used in low power modes. Add a build time warning when the GPT
timer is enabled, so that the user will be aware of the resolution
tradeoff when using the GPT timer.

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