soc: boards: arm: Refactor i.MX RT600 zephyr,flash usage Refactors the i.MX RT600 SoC series to be more consistent with the i.MX RT10xx SoC series by choosing a child node (external flash device) of the FlexSPI bus for zephyr,flash. Signed-off-by: Maureen Helm <maureen.helm@nxp.com>