doc: update reference to README.imximage

Update reference in many files detected by
scripts/documentation-file-ref-check

README.imximage => imx/mkimage/imximage.txt

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/board/tqc/tqma6/clocks.cfg b/board/tqc/tqma6/clocks.cfg
index a98b30b..1f2001c 100644
--- a/board/tqc/tqma6/clocks.cfg
+++ b/board/tqc/tqma6/clocks.cfg
@@ -3,7 +3,7 @@
  * Copyright (C) 2013 Boundary Devices
  * Copyright (C) 2013, 2014 Markus Niebel <Markus.Niebel@tq-group.com>
  *
- * Refer doc/README.imximage for more details about how-to configure
+ * Refer doc/imx/mkimage/imximage.txt for more details about how-to configure
  * and create imximage boot image
  */