Add an rpmsg driver sample, which demonstrates how to communicate with
an AMP-configured remote processor over the rpmsg bus.

Note how once probed, the driver can immediately start sending messages
using the rpmsg_send() API, without having to worry about creating endpoints
or allocating rpmsg addresses: all that work is done by the rpmsg bus,
and the required information is already embedded in the rpmsg channel
that the driver is probed with.

In this sample, the driver simply sends a "Hello World!" message to the remote
processor repeatedly.

Designed with Brian Swetland .

Signed-off-by: Ohad Ben-Cohen
Cc: Brian Swetland
Cc: Arnd Bergmann
Cc: Grant Likely
Cc: Tony Lindgren
Cc: Russell King
Cc: Rusty Russell
Cc: Andrew Morton
Cc: Greg KH
Cc: Stephen Boyd