of/fdt: Remove dead code and mark functions with __init

Some functions in here are never called, and others are only called
during __init. Remove the dead code and some dead exports for functions
that don't exist (I'm looking at you of_fdt_get_string!). Mark some
functions with __init so we can throw them away after we boot up and
poke at the FDT blob too.

Cc: Hsin-Yi Wang <hsinyi@chromium.org>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Rob Herring <robh@kernel.org>