mtd: rawnand: omap2: Allow build on K3 platforms

K3 platforms come with GPMC. Enable GPMC build for
K3 platforms.

Signed-off-by: Roger Quadros <rogerq@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20211209090458.24830-3-rogerq@kernel.org