boards: arm: Use DT_SIZE macros for nxp external memories

Refactors nxp i.mx, kinetis, and lpc board-level device trees to use
DT_SIZE_K and DT_SIZE_M macros to define external memory sizes. This is
self documenting and easier to read.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>