fdt: Add basic support for decoding GPIO definitions

This adds some support into fdtdec for reading GPIO definitions from
the fdt. We permit up to FDT_GPIO_MAX GPIOs in the system. Each GPIO
is of the form:

gpio-function-name = <phandle gpio_num flags>;

where:

phandle is a pointer to the GPIO node
gpio_num is the number of the GPIO (0 to 223)
flags is a flag, as follows:

   bit    meaning
   0      0=polarity normal, 1=active low (inverted)

An example is:

enable-propounder-gpios = <&gpio 43 0>;

which means that GPIO 43 is used to enable the propounder (setting the
GPIO high), or that you can detect that the propounder is enabled by
checking if the GPIO is high (the fdt does not indicate input/output).

Two main functions are provided:

fdtdec_decode_gpio() reads a GPIO property from an fdt node and decodes it
into a structure.

fdtdec_setup_gpio() sets up the GPIO by calling gpio_request for you.

Both functions can cope with the property being missing, which is taken to
mean that that GPIO function is not available or is not needed.

[For reference, from Stephen Warren <swarren@nvidia.com>. It may be that
we add this extra complexity later if needed:

The correct way to parse such a GPIO property in general is:

* Read the first cell.
* Find the node referenced by the phandle (the controller).
* Ensure property gpio-controller is present in the controller node.
* Read property #gpio-cells from the controller node.
* Extract #gpio-cells from the original property.
* Keep processing more cells from the original property; there may be
multiple GPIOs listed.

According to the binding documentation in the Linux kernel, Samsung
Exynos4 doesn't use this format, and while all other chips do have a
flags cell, about 50% of the controllers indicate the cell is unused.
]

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>