ARC: update the help message for CONFIG_ARC_BUILTIN_DTB_NAME

Commit abe11dde ("ARC: [plat-arcfpga]: Enabling DeviceTree for
Angel4 board") changed the default built-in DTB from "skeleton" to
"angel4".

Commit fd155792 ("ARC: [plat_arcfpga]->[plat_sim]") changed it
from "angel4" to "nsim_700".
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Vineet Gupta <vgupta@kernel.org>

arch/arc/Kconfig

@@ -553,7 +553,7 @@ config ARC_BUILTIN_DTB_NAME
 	string "Built in DTB"
 	help
 	  Set the name of the DTB to embed in the vmlinux binary
-	  Leaving it blank selects the minimal "skeleton" dtb
+	  Leaving it blank selects the "nsim_700" dtb.
 
 endmenu	 # "ARC Architecture Configuration"