fdt: Correct fdt_get_base_address()

This function appears to obtain the value of the 'ranges' property rather
than 'reg'. As such it does not behave as documented or expected.

In addition it picks up the second field of the property which is the size
(with prop += naddr) rather than the first which is the address.

Fix it.

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