gpio: sa1100: include <mach/generic.h>

sa1100_init_gpio() is declared in a machine specific header so it
can be called from platform code, but the definition is in the device
driver, which causes a warning:

drivers/gpio/gpio-sa1100.c:310:13: error: no previous prototype for 'sa1100_init_gpio' [-Werror=missing-prototypes]

It's already possible to include mach/generic.h from drivers, so add
this one here as well.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>