powerpc/iseries: Remove compiler version dependent hack

The creation of the flattened device tree depended on the compiler
putting the constant strings for an object in a section with a
particular name.  This was changed with recent compilers.  Do this
explicitly instead.

Without this patch, iseries kernels may silently not boot when built with
some compilers.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>