gpio: Remove unused asm/gpio.h files

asm/gpio.h is included only by linux/gpio.h, and then only when the arch
selects ARCH_HAVE_CUSTOM_GPIO_H.  Only the following arches select it: arm
avr32 blackfin m68k (COLDFIRE only) sh unicore32.

Remove the unused asm/gpio.h files for the arches that do not select
ARCH_HAVE_CUSTOM_GPIO_H.

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>