kbuild,tegra124: add dummy obj- for Kbuild

In Kbuild, every makefile must have non-empty obj- or obj-y.
Otherwise, built-in.o will not be created and the link stage
will fail.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

arch/arm/cpu/armv7/tegra124/Makefile

@@ -4,3 +4,6 @@
 #
 # SPDX-License-Identifier:     GPL-2.0+
 #
+
+# necessary to create built-in.o
+obj- := __dummy__.o