fdt: Include arch specific gpio.h instead of asm-generic/gpio.h

Include arch specific gpio.h instead of asm-generic/gpio.h
because several architectures (Microblaze, Blackfin, Nios2, OpenRISC)
define gpio functions in header file.
asm-generic/gpio.h can be included in arch specific gpio.h
(For example: ARM)

Signed-off-by: Michal Simek <monstr@monstr.eu>
CC: Simon Glass <sjg@chromium.org>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Simon Glass <sjg@chromium.org>