libfdt: Add helpers for accessing unaligned words

This adds some helpers to load (32 or 64 bit) words from an fdt blob, even
if they're unaligned and we're on a platform that doesn't like plain
unaligned loads and stores.  We then use the helpers in a number of places.
There are two purposes for this:

1) This makes libfdt more robust against a blob loaded at an unaligned
   address.  It's usually good practice to load a blob at a 64-bit
   alignment, but it's nice to work even then.

2) Users can use these helpers to load integer values from within property
   values.  These can often be unaligned, even if the blob as a whole is
   aligned, since some property encodings have integers and strings mixed
   together without any alignment gaps.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>