blackfin: extgpio: switch to gpiochip_add_data()

We're planning to remove the gpiochip_add() function to swith
to gpiochip_add_data() with NULL for data argument.

Cc: Sonic Zhang <sonic.zhang@analog.com>
Cc: Steven Miao <realmz6@gmail.com>
Cc: adi-buildroot-devel@lists.sourceforge.net
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>