ti_config_fragments: rpmsg: Enable PRUSS remoteproc support

Add support to build the PRUSS remoteproc drivers and a PRUSS
messaging driver - PRU RPMsg driver, as modules by default.
These will enable the PRU-ICSS present on various AM33xx, AM437x
and AM57xx family of SoCs.

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