Drop the '#interrupt-cells' property in the motherboard node which has no
effect as the node is neither an interrupt-controller or interrupt-map
(that's in the parent node).

Drop 'model' as it is not used by software nor documented.

Drop 'arm,v2m-memory-map' as it is not used by software. The purpose was
to describe which memory map, but that's all described by the DT
already.

Link: https://lore.kernel.org/r/20210819184239.1192395-4-robh@kernel.org
Cc: Andre Przywara <andre.przywara@arm.com>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>