powerpc: ppc4xx: Add a gpio.h header file

This is required at present for device tree control. The ppc4xx does support
GPIOs but does not seem to have a proper driver. So this file is empty.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/powerpc/include/asm/arch-ppc4xx/gpio.h

+/*
+ * (C) Copyright 2014 Google, Inc
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+/* This is empty for now as we don't support the generic GPIO interface */