boards: soc: arm: Set zephyr,itcm chosen node for i.MX RT boards

Sets the device tree chosen node for instruction tightly coupled memory
(ITCM) on all i.MX RT boards. Leverages the common Cortex-M linker
section instead of the SoC-specific one.

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