mailbox: Add support for Qualcomm IPCC

Add support for the Inter-Processor Communication Controller (IPCC)
block from Qualcomm that coordinates the interrupts (inbound & outbound)
for Multiprocessor (MPROC), COMPUTE-Level0 (COMPUTE-L0) & COMPUTE-Level1
(COMPUTE-L1) protocols for the Application Processor Subsystem (APSS).

This driver is modeled as an irqchip+mailbox driver. The irqchip part
helps in receiving the interrupts from the IPCC clients such as modems,
DSPs, PCI-E etc... and forwards them to respective entities in APSS.

On the other hand, the mailbox part is used to send interrupts to the IPCC
clients from the entities of APSS.

Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>
Signed-off-by: Venkata Narendra Kumar Gutta <vnkgutta@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
[mani: moved to mailbox, added static mbox channels and cleanups]
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>