dt-bindings: mailbox: qcom-ipcc: Document QCS8300 IPCC

Document the Inter-Processor Communication Controller on the Qualcomm
QCS8300 Platform, which will be used to route interrupts across various
subsystems found on the SoC.

Signed-off-by: Jingyi Wang <quic_jingyw@quicinc.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>