dtc: Enable and fix -Wcast-qual warnings

Enabling -Wcast-qual warnings in dtc shows up a number of places where
we are incorrectly discarding a const qualification.  There are also
some places where we are intentionally discarding the 'const', and we
need an ugly cast through uintptr_t to suppress the warning.  However,
most of these are pretty well isolated with the *_w() functions.  So
in the interests of maximum safety with const qualifications, this
patch enables the warnings and fixes the existing complaints.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
diff --git a/libfdt/libfdt.h b/libfdt/libfdt.h
index 8645de0..ad58a84 100644
--- a/libfdt/libfdt.h
+++ b/libfdt/libfdt.h
@@ -125,7 +125,7 @@
 const void *fdt_offset_ptr(const void *fdt, int offset, int checklen);
 static inline void *fdt_offset_ptr_w(void *fdt, int offset, int checklen)
 {
-	return (void *)fdt_offset_ptr(fdt, offset, checklen);
+	return (void *)(uintptr_t)fdt_offset_ptr(fdt, offset, checklen);
 }
 
 uint32_t fdt_next_tag(const void *fdt, int offset, int *nextoffset);
@@ -375,8 +375,8 @@
 						      const char *name,
 						      int *lenp)
 {
-	return (struct fdt_property *)fdt_get_property(fdt, nodeoffset,
-						       name, lenp);
+	return (struct fdt_property *)(uintptr_t)
+		fdt_get_property(fdt, nodeoffset, name, lenp);
 }
 
 /**
@@ -411,7 +411,7 @@
 static inline void *fdt_getprop_w(void *fdt, int nodeoffset,
 				  const char *name, int *lenp)
 {
-	return (void *)fdt_getprop(fdt, nodeoffset, name, lenp);
+	return (void *)(uintptr_t)fdt_getprop(fdt, nodeoffset, name, lenp);
 }
 
 /**