common: Make sure arch-specific map_sysmem() is defined

In the case where the arch defines a custom map_sysmem(), make sure that
including just mapmem.h is sufficient to have these functions as they
are when the arch does not override it.

Also split the non-arch specific functions out of common.h

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/common/cmd_fdt.c b/common/cmd_fdt.c
index 48b3e70..682b655 100644
--- a/common/cmd_fdt.c
+++ b/common/cmd_fdt.c
@@ -15,6 +15,7 @@
 #include <asm/global_data.h>
 #include <libfdt.h>
 #include <fdt_support.h>
+#include <mapmem.h>
 #include <asm/io.h>
 
 #define MAX_LEVEL	32		/* how deeply nested we will go */