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>