boards: arm: Enable FlexSPI driver on mimxrt685_evk.

Enable FlexSPI NOR flash driver with XIP build options.
Configure FlexSPI pins, update board documentation.
Add FlexSPI NOR flash dts node for mimxrt685_evk.

Enable flash storage by adding partition to dts file.
Using last 1MB for storage, and reserving 63MB for code.

Signed-off-by: Saurabh Jagdhane <saurabh.jagdhane@nxp.com>