kbuild: use SoC-specific CONFIG to descend into SoC directory

Use "obj-$(CONFIG_FOO) += foo/" where it is possible.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/arch/arm/cpu/arm720t/Makefile b/arch/arm/cpu/arm720t/Makefile
index 9893b08..9f61ea2 100644
--- a/arch/arm/cpu/arm720t/Makefile
+++ b/arch/arm/cpu/arm720t/Makefile
@@ -9,5 +9,7 @@
 obj-y	= interrupts.o cpu.o
 
 obj-$(CONFIG_TEGRA) += tegra-common/
-
-obj-y += $(if $(SOC),$(SOC)/)
+obj-$(CONFIG_TEGRA20) += tegra20/
+obj-$(CONFIG_TEGRA30) += tegra30/
+obj-$(CONFIG_TEGRA114) += tegra114/
+obj-$(CONFIG_TEGRA124) += tegra124/