arm: mach-k3: am642: Define NAND boot device

AM642 SoC supports booting from GPMC NAND device.
Define boot device for it.

Signed-off-by: Roger Quadros <rogerq@kernel.org>
Link: https://www.mail-archive.com/u-boot@lists.denx.de/msg499180.html


Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>