of/fdt: fix sparse warning

this patch fixes following sparse warning:

fdt.c:765:12: warning: symbol 'early_init_dt_scan_chosen_serial' was not declared. Should it be static?

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>