ti_config_fragments: rpmsg: Enable rpmsg-rpc driver

Add support to build the RPMsg bus/client driver rpmsg-rpc to
enable the MultiMedia hardware accelerated stack on DRA7xx/AM57xx
SoCs. The driver is currently supported only for OMAP remoteprocs
and depends on the OMAP remoteproc driver being enabled.

Signed-off-by: Suman Anna <s-anna@ti.com>