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/fs/fs.c b/fs/fs.c
index 483273f..ac0897d 100644
--- a/fs/fs.c
+++ b/fs/fs.c
@@ -17,6 +17,7 @@
 #include <config.h>
 #include <errno.h>
 #include <common.h>
+#include <mapmem.h>
 #include <part.h>
 #include <ext4fs.h>
 #include <fat.h>