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.

Signed-off-by: Sibi Sankar <quic_sibis@quicinc.com>
Signed-off-by: Abel Vesa <abel.vesa@linaro.org>