gpiolib: remove asm-generic/gpio.h

The asm-generic/gpio.h file is now always included when
using gpiolib, so just move its contents into linux/gpio.h
with a few minor simplifications.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>