gpio: delete AB8500 driver

The AB8500 GPIO driver has been marked BROKEN for ages, and we
have something better in store: a shiny new pinctrl driver. So
let use delete this old driver as the first step.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

diff --cc drivers/gpio/Kconfig
Simple merge

diff --cc drivers/gpio/Makefile
Simple merge

diff --cc drivers/gpio/gpio-ab8500.c
Simple merge