boards: mimxrt1xxx: Added support for GPT hardware timer on RT1xxx

Adds support for using GPT as the hardware timer source (as opposed to
systick) for all RT1xxx platforms. This requires moving the clock
frequency of these devices into a defconfig so it can be overridden by
the GPT clock frequency.

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