kconfig: add CONFIG_SUPPORT_SPL

CONFIG_SPL should not be enabled for boards that do not have SPL.
CONFIG_SUPPORT_SPL introduced by this commit should be "select"ed
by boards with SPL support and CONFIG_SPL should depend on it.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/powerpc/cpu/mpc5xxx/Kconfig b/arch/powerpc/cpu/mpc5xxx/Kconfig
index 8a477e7..6a2f5e5 100644
--- a/arch/powerpc/cpu/mpc5xxx/Kconfig
+++ b/arch/powerpc/cpu/mpc5xxx/Kconfig
@@ -9,6 +9,7 @@
 
 config TARGET_A3M071
 	bool "Support a3m071"
+	select SUPPORT_SPL
 
 config TARGET_A4M072
 	bool "Support a4m072"