libfdt: Make fdt_string() return a const pointer

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> Currently, fdt_string() returns a (non-const) char *, despite taking a
> const void *fdt.  This is inconsistent with all the other read-only
> functions which all return const pointers into the blob.
> 
> This patch fixes that.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

jdl