MIPS: Make CM GCR base configurable

Without adding a prompt for CONFIG_MIPS_CM_BASE, Kconfig doesn't allow
defconfigs to set it. Provide the prompt in order to allow for that.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/mips/lib/cache.c b/arch/mips/lib/cache.c
index bd14ba6..91b037f 100644
--- a/arch/mips/lib/cache.c
+++ b/arch/mips/lib/cache.c
@@ -7,7 +7,9 @@
 
 #include <common.h>
 #include <asm/cacheops.h>
+#ifdef CONFIG_MIPS_L2_CACHE
 #include <asm/cm.h>
+#endif
 #include <asm/mipsregs.h>
 
 DECLARE_GLOBAL_DATA_PTR;