boards: nxp: frdm_mcxn947: migrate ov7670 to a shield

The OV7670 was defined as part of the FRDM-MCXN947 board attached to the
SmartDMA and I2C peripheral. Migrate the devicetree to a reusable shield
that can be swapped with other image sensors.

Signed-off-by: Josuah Demangeon <me@josuah.net>