Add generic gpio.h in asm-generic

Since we want want to have a standard GPIO interface, this adds a definition
for this into include/asm-generic/gpio.h.

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

include/asm-generic/gpio.h

+/*
+ * Copyright (c) 2011 The Chromium OS Authors.
+ * See file CREDITS for list of people who contributed to this
+ * project.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of
+ * the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston,
+ * MA 02111-1307 USA
+ */
+
+/*
+ * Generic GPIO API for U-Boot
+ *
+ * GPIOs are numbered from 0 to GPIO_COUNT-1 which value is defined
+ * by the SOC/architecture.
+ *
+ * Each GPIO can be an input or output. If an input then its value can
+ * be read as 0 or 1. If an output then its value can be set to 0 or 1.
+ * If you try to write an input then the value is undefined. If you try
+ * to read an output, barring something very unusual,  you will get
+ * back the value of the output that you previously set.
+ *
+ * In some cases the operation may fail, for example if the GPIO number
+ * is out of range, or the GPIO is not available because its pin is
+ * being used by another function. In that case, functions may return
+ * an error value of -1.
+ */
+
+/**
+ * Make a GPIO an input.
+ *
+ * @param gp	GPIO number
+ * @return 0 if ok, -1 on error
+ */
+int gpio_direction_input(int gp);
+
+/**
+ * Make a GPIO an output, and set its value.
+ *
+ * @param gp	GPIO number
+ * @param value	GPIO value (0 for low or 1 for high)
+ * @return 0 if ok, -1 on error
+ */
+int gpio_direction_output(int gp, int value);
+
+/**
+ * Get a GPIO's value. This will work whether the GPIO is an input
+ * or an output.
+ *
+ * @param gp	GPIO number
+ * @return 0 if low, 1 if high, -1 on error
+ */
+int gpio_get_value(int gp);
+
+/**
+ * Set an output GPIO's value. The GPIO must already be an output of
+ * this function may have no effect.
+ *
+ * @param gp	GPIO number
+ * @param value	GPIO value (0 for low or 1 for high)
+ * @return 0 if ok, -1 on error
+ */
+int gpio_set_value(int gp, int value);