ARC: update the help message for CONFIG_ARC_BUILTIN_DTB_NAME
Commit abe11ddea1d7 ("ARC: [plat-arcfpga]: Enabling DeviceTree for
Angel4 board") changed the default built-in DTB from "skeleton" to
"angel4".
Commit fd1557923b2e ("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>