rpi4/fdt: Move dtb_size() function to fdt_wrappers.h

Getting the actual size of a DTB blob is useful beyond the Raspberry Pi
port, so let's move this helper to a common header.

Change-Id: Ia5be46e9353ca859a1e5ad9e3c057a322dfe22e2
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/include/common/fdt_wrappers.h b/include/common/fdt_wrappers.h
index 1d8da18..a571092 100644
--- a/include/common/fdt_wrappers.h
+++ b/include/common/fdt_wrappers.h
@@ -9,6 +9,8 @@
 #ifndef FDT_WRAPPERS_H
 #define FDT_WRAPPERS_H
 
+#include <libfdt_env.h>
+
 /* Number of cells, given total length in bytes. Each cell is 4 bytes long */
 #define NCELLS(len) ((len) / 4U)
 
@@ -37,4 +39,11 @@
 uint64_t fdtw_translate_address(const void *dtb, int bus_node,
 				uint64_t base_address);
 
+static inline uint32_t fdt_blob_size(const void *dtb)
+{
+	const uint32_t *dtb_header = dtb;
+
+	return fdt32_to_cpu(dtb_header[1]);
+}
+
 #endif /* FDT_WRAPPERS_H */