State the 64-bit alignment requirement earlier

This is currently buried in a paragraph later in the Alignment section.
State the requirement up front, along with a note about existing uses.

Series-to: dtc
Series-cc: rob

Signed-off-by: Simon Glass <[email protected]>

Author: sjg20

source/chapter5-flattened-format.rst

@@ -390,6 +390,10 @@ offset from the beginning of the devicetree blob.
 Alignment
 ---------
 
+The devicetree blob shall be located at an 8-byte-aligned address. To maintain
+backwards compatibilty for 32-bit machines, 4-byte alignment is supported by
+some software, but this is not |spec|-compliant.
+
 For the data in the memory reservation and structure blocks to be used
 without unaligned memory accesses, they shall lie at suitably aligned
 memory addresses. Specifically, the memory reservation block shall be