dt-bindings: mailbox: zynqmp_ipi: fix unit address

Fix unit address to match the first address specified in the reg
property of the node in example.

Signed-off-by: Kangmin Park <l4stpr0gr4m@gmail.com>
Link: https://lore.kernel.org/r/20200625135158.5861-1-l4stpr0gr4m@gmail.com


Signed-off-by: Rob Herring <robh@kernel.org>