soc: nxp_imx: Add support for device configuration data (DCD)

Adds support for the device configuration data (DCD), which provides a
sequence of commands to the imx rt boot ROM to initialize components
such as an SDRAM.

It is now possible to use the external SDRAM instead of the internal
DTCM on the mimxrt1020_evk, mimxrt1050_evk, and mimxrt1060_evk. Note,
however, that the default board configurations still link data into
internal DTCM, therefore you must use a device tree overlay to override
"zephyr,sram = &sdram0"

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