mailbox: zynqmp: Fix counts of child nodes

If child mailbox node status is disabled it causes
crash in interrupt handler. Fix this by assigning
only available child node during driver probe.

Fixes: 4981b82b

 ("mailbox: ZynqMP IPI mailbox controller")
Signed-off-by: Tanmay Shah <tanmay.shah@amd.com>
Acked-by: Michal Simek <michal.simek@amd.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20230311012407.1292118-2-tanmay.shah@amd.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>