drivers: firmware: Add support for IRONside calls

IRONside calls are remote procedure calls which comprise the runtime
interface of Nordic IRONside SE. They are realized using a simple IPC
mechanism.

A local domain (client) issues requests to the server by exchanging data
in shared memory, which is divided into evenly sized buffers. The client
selects a buffer, writes a request into it, and sends it to the server.
The server processes that request and writes a response into the same
buffer before returning it to the client.

This patch adds the initial client-side implementation on top of MBOX.
It features cache management and a blocking alloc/dispatch/release API
for synchronous, zero-copy transfers.

A new devicetree binding is added to support this implementation. It is
patterned after the `zephyr,ipc-*` bindings, where each node associates
a pair of mailboxes and a shared memory region.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>