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/drivers/demo/demo-simple.c b/drivers/demo/demo-simple.c
index 2bcb7df..f069748 100644
--- a/drivers/demo/demo-simple.c
+++ b/drivers/demo/demo-simple.c
@@ -10,6 +10,7 @@
 #include <common.h>
 #include <dm.h>
 #include <dm-demo.h>
+#include <mapmem.h>
 #include <asm/io.h>
 
 static int simple_hello(struct udevice *dev, int ch)