gpio: Include linux/gpio.h instead of asm/gpio.h

Most arches have an asm/gpio.h that merely includes linux/gpio.h.  The
others select ARCH_HAVE_CUSTOM_GPIO_H, and when that's selected,
linux/gpio.h includes asm/gpio.h.

Therefore, code should include linux/gpio.h instead of including asm/gpio.h
directly.

Remove includes of asm/gpio.h, adding an include of linux/gpio.h when
necessary.

This is a follow-on to 7563bbf8 ("gpiolib/arches: Centralise
bolierplate asm/gpio.h").

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>