arm: fdt_control: fix a build error with CONFIG_OF_EMBED=y

The build fails if a non-generic ARM board is compiled
with CONFIG_OF_EMBED=y.

The correct symbol name for embedded FDT is not __dtb_db_begin,
but __dtb_dt_begin. (A typo introduced by commit 6ab6b2af)

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>