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>
Acked-by: Gerald Van Baren <vanbaren@cideas.com>
diff --git a/libfdt/fdt_ro.c b/libfdt/fdt_ro.c
index 8382afd..1c897c5 100644
--- a/libfdt/fdt_ro.c
+++ b/libfdt/fdt_ro.c
@@ -81,7 +81,7 @@
 
 const char *fdt_string(const void *fdt, int stroffset)
 {
-	return (char *)fdt + fdt_off_dt_strings(fdt) + stroffset;
+	return (const char *)fdt + fdt_off_dt_strings(fdt) + stroffset;
 }
 
 int fdt_get_mem_rsv(const void *fdt, int n, uint64_t *address, uint64_t *size)