rpmsg-kdrv: add access APIs from virtual drivers

rpmsg-kdrv framework collects the info about all
available virtual devices / channels from remote
cores and probes the corresponding driver.

Based on the device class (display / network etc)
the client drivers must make appropriate calls to
make use of the available services.

Adding a demo device layer (exposed by remote-device
DEMO_DEVICE class) with APIs for
* sending ping requests (demonstrates REQUEST / RESPONSE)
* sending c2s messages (demonstrates client-to-server
  one-shot messages)
* registering callback for s2c messages (demonstrates
  server-to-client events)

Tested-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Subhajit Paul <subhajit_paul@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>