dtc: Make many functions 'static'

This patch marks various functions not shared between c files
'static', as they should be.  There are a couple of functions in dtc,
and many in the testsuite.

This is *almost* enough to enable the -Wmissing-prototypes warning.
It's not quite enough, because there's a mess of junk in the flex
generated code which triggers that warning which I'm not yet sure how
to deal with.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

tests/path_offset.c

@@ -28,7 +28,7 @@
 #include "tests.h"
 #include "testdata.h"
 
-int check_subnode(void *fdt, int parent, const char *name)
+static int check_subnode(void *fdt, int parent, const char *name)
 {
 	int offset;
 	const struct fdt_node_header *nh;