doc: Move device tree bindings documentation to doc/device-tree-bindings

Currently the U-Boot project contains 2 documentation directories:

- doc/
- Documentation/

The Documentation directory only contains device tree bindings related
content, so move the 3 files to doc/device-tree-bindings/.

Signed-off-by: Breno Lima <breno.lima@nxp.com>

diff --git a/Documentation/devicetree/bindings/phy/no-op.txt b/doc/device-tree-bindings/phy/no-op.txt
similarity index 100%
rename from Documentation/devicetree/bindings/phy/no-op.txt
rename to doc/device-tree-bindings/phy/no-op.txt

diff --git a/Documentation/devicetree/bindings/phy/phy-stm32-usbphyc.txt b/doc/device-tree-bindings/phy/phy-stm32-usbphyc.txt
similarity index 100%
rename from Documentation/devicetree/bindings/phy/phy-stm32-usbphyc.txt
rename to doc/device-tree-bindings/phy/phy-stm32-usbphyc.txt

diff --git a/Documentation/devicetree/bindings/rtc/brcm,brcmstb-waketimer.txt b/doc/device-tree-bindings/rtc/brcm,brcmstb-waketimer.txt
similarity index 100%
rename from Documentation/devicetree/bindings/rtc/brcm,brcmstb-waketimer.txt
rename to doc/device-tree-bindings/rtc/brcm,brcmstb-waketimer.txt