gpiolib: drop irq_base field from gpio_chip struct

Hence, the last user of irq_base field was removed by commit b4c495f0
("gpio: mockup: use irq_sim") it can be removed safely.

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