memory: omap-gpmc: use devm_gpiochip_add_data()

This saves a few codelines in the driver.

Cc: Roger Quadros <rogerq@ti.com>
Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Roger Quadros <rogerq@ti.com>