kbuild: use boolean macros to select tegra*-common directory

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

arch/arm/cpu/Makefile

@@ -1,3 +1,6 @@
 obj-$(CONFIG_AT91FAMILY) += at91-common/
-obj-$(CONFIG_TEGRA) += $(SOC)-common/
+obj-$(CONFIG_TEGRA20) += tegra20-common/
+obj-$(CONFIG_TEGRA30) += tegra30-common/
+obj-$(CONFIG_TEGRA114) += tegra114-common/
+obj-$(CONFIG_TEGRA124) += tegra124-common/
 obj-$(CONFIG_TEGRA) += tegra-common/