soc: qcom: ipcc: Add support for IPCC controller

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

As a part of its inbound communication, the driver would be responsible
for forwarding the interrupts from various clients, such as Modem, DSPs,
PCIe, and so on, to the entities running on the APPS. As a result, it's
implemented as an irq_chip driver.

On the other hand, the driver also registers as a mailbox controller
that provides a mailbox interface to interrupt other clients connected
to the IPCC, acting as an outbound communication channel.

Change-Id: I0abdefaa3804a00f35a536b3291eac514cfe9ab2
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>