dt-bindings: arm,vexpress-juno: Fix 'motherboard' node name

Commit 9d0a36dd ("arm64: dts: fvp/juno: Fix bus node names") changed
'motherboard' to 'motherboard-bus', but didn't update the schema. In the
mean time, the simple-bus schema started requiring child nodes to have a
unit-address. Update the schema to address both of these issues.

Cc: Andre Przywara <andre.przywara@arm.com>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210819184239.1192395-2-robh@kernel.org