mtd: rawnand: Fix misuses of of_match_node()

On non-OF enabled platforms (CONFIG_OF is not set), of_match_node() will
expand to NULL. The of_device_id array pointed by the macro will then be
left unused. Let's mark the array __maybe_unused in this case to prevent
compiler warnings.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/linux-mtd/20220127110802.1064963-1-miquel.raynal@bootlin.com