ARM: UniPhier: add dummy gpio.h to enable CONFIG_OF_CONTROL

If CONFIG_OF_CONTROL is enabled, lib/fdtdec.c is compiled.
It includes <asm/gpio.h> and then <asm/gpio.h> includes
<asm/arch/gpio.h>.  Consequently, all the SoCs that enable
CONFIG_OF_CONTROL must have <asm/arch/gpio.h> even if they do not
support GPIO.

In the first place, GPIO has nothing to do with OF_CONTROL.
It is wrong that lib/fdtdec.c includes GPIO functions; it should
be split into two files, FDT-common things and GPIO things.
It is, however, a pretty big work to fix that correctly.

This is a compromised commit to add a dummy <asm/arch/gpio.h>
to support OF_CONTROL for UniPhier platform.  This dummy header
will be removed after FDT-GPIO stuff is fixed correctly.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Reviewed-by: Simon Glass <sjg@chromium.org>