asm-generic/gpio.h: add some forward decls of the device struct

After the recent commit a4177ee7, attempting to include asm-generic/gpio.h
in otherwise "slim" code results in ugly warnings like so:

  CC      arch/blackfin/kernel/bfin_gpio.o
In file included from arch/blackfin/include/asm/gpio.h:278,
                 from arch/blackfin/kernel/bfin_gpio.c:15:
include/asm-generic/gpio.h:193: warning:
‘struct device’ declared inside parameter list
its scope is only this definition or declaration, which is probably not what you want

So add simple C forward decls of the struct device to avoid these.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>