gpio: davinci: Remove unused member of davinci_gpio_controller

This was added as part of the patch in the fixes below, but was
not needed or used, remove this here.

Fixes: 8e11047b

 ("gpio: davinci: Add support for multiple GPIO controllers")
Tested-by: Keerthy <j-keerthy@ti.com>
Acked-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>