memory: omap-gpmc: add error message if bank-width property is absent

Instead of failing silently log a hint for the dt author about the
reason of the failure.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>