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: NMasahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: NSimon Glass <sjg@chromium.org>

arch/arm/lib/board.c

@@ -277,7 +277,7 @@ void board_init_f(ulong bootflag)
 	gd->mon_len = (ulong)&__bss_end - (ulong)_start;
 #ifdef CONFIG_OF_EMBED
 	/* Get a pointer to the FDT */
-	gd->fdt_blob = __dtb_db_begin;
+	gd->fdt_blob = __dtb_dt_begin;
 #elif defined CONFIG_OF_SEPARATE
 	/* FDT is at end of image */
 	gd->fdt_blob = &_end;