fdt: Rename setup_fdt() and make it prepare also

There is little reason to split these two functions. Bring them together
which simplifies the init sequence.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/fdtdec.h

@@ -797,6 +797,6 @@ int fdtdec_decode_memory_region(const void *blob, int node,
 /**
  * Set up the device tree ready for use
  */
-int setup_fdt(void);
+int fdtdec_setup(void);
 
 #endif