mailbox: zynqmp: setup IPI for each valid child node

As per zynqmp-ipi bindings, zynqmp IPI node can have multiple child nodes.
Current IPI setup function is set only for first child node. If IPI node
has multiple child nodes in the device-tree, then IPI setup fails for
child nodes other than first child node. In such case kernel will crash.
Fix this crash by registering IPI setup function for each available child
node.

Signed-off-by: Tanmay Shah <tanmay.shah@amd.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>