fdt: add decode helper library

This library provides useful functions to drivers which want to use
the fdt to control their operation. Functions are provided to:

- look up and enumerate a device type (for example assigning i2c bus 0,
     i2c bus 1, etc.)
- decode basic types from the fdt, like addresses and integers

While this library is not strictly necessary, it helps to minimise the
changes to a driver, in order to make it work under fdt control. Less
code is required, and so the barrier to switch drivers over is lower.

Additional functions to read arrays and GPIOs could be made available
here also.

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