arm64: dts: arm/juno: Drop erroneous 'mbox-name' property

The 'mbox-name' property in the Juno mailbox node is undocumented and
unused. It's the consumer side of the mailbox binding that have
'mbox-names' properties.

Link: https://lore.kernel.org/r/20220610213308.2288094-1-robh@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>