fdt: Add functions to access phandles, arrays and bools

Add a function to look up a property which is a phandle in a node, and
another to read a fixed-length integer array from an fdt property.
Also add a function to read boolean properties, although there is no
actual boolean type in U-Boot.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Gerald Van Baren <vanbaren@cideas.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>