docs: Update DTB format references

There were two references to devicetree/booting-without-of.rst (which has
been removed) for DTB format information, and
devicetree/usage-model.rst pointed to
https://elinux.org/Device_Tree_Usage. Change all three of these references to
https://www.devicetree.org/specifications/.

Signed-off-by: Milan Lakhani <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/devicetree/usage-model.rst

···
 the device tree data format can be found on the device tree usage page
 at devicetree.org\ [1]_.
 
-.. [1] https://elinux.org/Device_Tree_Usage
+.. [1] https://www.devicetree.org/specifications/
 
 The "Open Firmware Device Tree", or simply Device Tree (DT), is a data
 structure and language for describing hardware.  More specifically, it

Documentation/translations/zh_CN/arm/Booting

···
 
 bootloader 必须以 64bit 地址对齐的形式加载一个设备树映像(dtb)到系统
 RAM 中，并用启动数据初始化它。dtb 格式在文档
-Documentation/devicetree/booting-without-of.rst 中。内核将会在
+https://www.devicetree.org/specifications/ 中。内核将会在
 dtb 物理地址处查找 dtb 魔数值（0xd00dfeed），以确定 dtb 是否已经代替
 标签列表被传递进来。