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/arm920t/Makefile b/arch/arm/cpu/arm920t/Makefile
index 9f885ff..a72e5de 100644
--- a/arch/arm/cpu/arm920t/Makefile
+++ b/arch/arm/cpu/arm920t/Makefile
@@ -10,4 +10,9 @@
 obj-y	+= cpu.o
 obj-$(CONFIG_USE_IRQ)	+= interrupts.o
 
-obj-y += $(if $(SOC),$(SOC)/)
+obj-$(if $(filter a320,$(SOC)),y) += a320/
+obj-$(CONFIG_AT91FAMILY) += at91/
+obj-$(CONFIG_EP93XX) += ep93xx/
+obj-$(CONFIG_IMX) += imx/
+obj-$(CONFIG_KS8695) += ks8695/
+obj-$(CONFIG_S3C24X0) += s3c24x0/