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

Sets the device tree chosen node for data tightly coupled memory (DTCM)
on i.mx rt boards that aren't already using DTCM as the chosen SRAM.
Leverages the common cortex-m linker section instead of the soc-specific
one.

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