boards: arm: mimxrt685_evk: Update write alignment and partition sizes

Now that MCUBoot supports 16 byte alignment, the 1 byte write mode
is no longer neccesary for the RT685. Partition sizes have been
reduced, as MCUBoot swap times are otherwise abnormally long.

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