ti_config_fragments: rpmsg: Enable PRUSS support

Add support to build the PRUSS platform drivers including the PRUSS
INTC IrqChip driver, PRU remoteproc driver and a PRUSS messaging
driver - PRU RPMsg driver, as modules by default. This will enable
the PRU-ICSS instance(s) present on various AM33xx, AM437x, AM57xx
and 66AK2G family of SoCs.

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