gpio: omap: drop dev field from gpio_bank structure

GPIO chip structure already has "parent" field which is used for the
same purpose as "dev" field in gpio_bank structure - store pointer on
GPIO device.

Hence, drop duplicated "dev" field from gpio_bank structure.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Santosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>