of/fdt: Fix #ifdef dependency of early flattree declarations

If OF_FLATTREE=y, but OF_EARLY_FLATTREE=n:

    drivers/tty/serial/earlycon.o: In function `param_setup_earlycon':
    earlycon.c:(.init.text+0x3a4): undefined reference to `early_init_dt_scan_chosen_stdout'

Fix this by moving the early flattree forward declarations and dummy
implementations inside an #ifdef CONFIG_OF_EARLY_FLATTREE block.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>