boards: arm: nxp: add documentation to RT series about external memories

Add documentation to NXP i.MX RT series SOCs about external memories
present on the board. This is intended to help highlight how the SOC
enables external memory using configuration blocks exposed to the
bootrom.

Addtionally, add a check to these boards to warn a user if they are
attempting to utilize SDRAM as their RAM bank without a DCD present.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>