arm: Move fdt check earlier so that board_early_init_f() can use it

We want to use the fdt inside board_early_init_f(), so check for its
presence earlier in the pre-reloc init sequence.

So far ARM and microblaze are the only only ones that use
CONFIG_OF_CONTROL. Microblaze does not have the same init loop, and in
particular does not have the board_early_init_f() call. So a patch for
microblaze would have no meaning.

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