kbuild: Descend into SOC directory from CPU directory

Some CPUs of some architectures have SOC directories.
At present, the build system directly descends into SOC directories
from the top Makefile, but it should generally descend into each
directory from its parent directory.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/arch/arm/cpu/armv7/Makefile b/arch/arm/cpu/armv7/Makefile
index afeed4d..7d8ca08 100644
--- a/arch/arm/cpu/armv7/Makefile
+++ b/arch/arm/cpu/armv7/Makefile
@@ -37,3 +37,5 @@
 ifneq (,$(filter s5pc1xx exynos,$(SOC)))
 obj-y += s5p-common/
 endif
+
+obj-y += $(if $(SOC),$(SOC)/)