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>

boards/arm/mimxrt1020_evk/mimxrt1020_evk.dts

@@ -18,6 +18,7 @@
 	};
 
 	chosen {
+		zephyr,sram = &sdram0;
 		zephyr,console = &lpuart1;
 		zephyr,shell-uart = &lpuart1;
 	};