boards: soc: arm: Set zephyr,sram chosen node for i.mx rt boards

Removes the DATA_LOCATION Kconfig symbol from the i.mx rt soc series and
refactors corresponding boards to use a device tree chosen node instead.
The external SDRAM is chosen on all boards that can support it;
otherwise the internal DTCM is chosen.

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