dm: fdt: Add a function to decode phandles with arguments

For GPIOs and other functions we want to look up a phandle and then decode
a list of arguments for that phandle. Each phandle can have a different
number of arguments, specified by a property in the target node. This is
the "#gpio-cells" property for GPIOs.

Add a function to provide this feature, taken modified from Linux 3.18.

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