dt-bindings: arm: Add missing root node constraint for board/SoC bindings

dt-bindings: arm: Add missing root node constraint for board/SoC bindings

Board/SoC top-level compatible bindings should be constrained to the root
node. Add the missing constraints that the node name must be "/".

Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Imre Kaloz <kaloz@openwrt.org>
Cc: Krzysztof Halasa <khalasa@piap.pl>
Cc: Andy Gross <agross@kernel.org>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Jonathan Hunter <jonathanh@nvidia.com>
Cc: Dinh Nguyen <dinguyen@kernel.org>
Cc: Anders Berg <anders.berg@lsi.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Alexandre TORGUE <alexandre.torgue@st.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20201001200943.1193870-1-robh@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>