This introduces a new rpmsg backend for the Qualcomm SMD system,
allowing communication with various remote processors found in Qualcomm
platforms. The implementation is based on, and intends to replace,
drivers/soc/qcom/smd.c with the necessary adaptions for fitting with the
rpmsg core.

Based on original work by Sricharan R <sricharan@codeaurora.org>

Cc: Sricharan R <sricharan@codeaurora.org>
Signed-off-by: NBjorn Andersson <bjorn.andersson@linaro.org>

Extract the now indirect rpmsg_create_ept() interface to a separate
file and start building up a rpmsg core.
Signed-off-by: NBjorn Andersson <bjorn.andersson@linaro.org>

Add a virtio-based inter-processor communication bus, which enables
kernel drivers to communicate with entities, running on remote
processors, over shared memory using a simple messaging protocol.

Every pair of AMP processors share two vrings, which are used to send
and receive the messages over shared memory.

The header of every message sent on the rpmsg bus contains src and dst
addresses, which make it possible to multiplex several rpmsg channels on
the same vring.

Every rpmsg channel is a device on this bus. When a channel is added,
and an appropriate rpmsg driver is found and probed, it is also assigned
a local rpmsg address, which is then bound to the driver's callback.

When inbound messages carry the local address of a bound driver,
its callback is invoked by the bus.

This patch provides a kernel interface only; user space interfaces
will be later exposed by kernel users of this rpmsg bus.

Designed with Brian Swetland <swetland@google.com>.
Signed-off-by: NOhad Ben-Cohen <ohad@wizery.com>
Acked-by: Rusty Russell <rusty@rustcorp.com.au> (virtio_ids.h)
Cc: Brian Swetland <swetland@google.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Greg KH <greg@kroah.com>
Cc: Stephen Boyd <sboyd@codeaurora.org>