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/mips/cpu/mips32/Makefile b/arch/mips/cpu/mips32/Makefile
index 1221b9e..fa82dd3 100644
--- a/arch/mips/cpu/mips32/Makefile
+++ b/arch/mips/cpu/mips32/Makefile
@@ -9,4 +9,4 @@
 obj-y	= cache.o
 obj-y	+= cpu.o interrupts.o time.o
 
-obj-y += $(if $(SOC),$(SOC)/)
+obj-$(CONFIG_SOC_AU1X00) += au1x00/