mailbox: Add support for QTI CPUCP mailbox controller

Add support for CPUSS Control Processor (CPUCP) mailbox controller,
this driver enables communication between AP and CPUCP by acting as
a doorbell between them.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Sibi Sankar <quic_sibis@quicinc.com>
Reviewed-by: Bjorn Andersson <andersson@kernel.org>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>