mailbox: zynqmp-ipi: Remove dev.parent check in zynqmp_ipi_free_mboxes

The ipi_mbox->dev.parent check is unreliable proxy for registration
status as it fails to protect against probe failures that occur after
the parent is assigned but before device_register() completes.

device_is_registered() is the canonical and robust method to verify the
registration status.

Remove ipi_mbox->dev.parent check in zynqmp_ipi_free_mboxes().

Fixes: 4981b82b ("mailbox: ZynqMP IPI mailbox controller")
Signed-off-by: Harini T <harini.t@amd.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>