dt-bindings: soc: ti: Update TI PRUSS bindings regarding clock-muxes

ICSS/ICSSG modules have an IEP clock mux that allow selection of
internal IEP clock from 2 clock sources.

ICSSG module has a CORE clock mux that allows selection of internal CORE
clock from 2 clock sources.

Add binding information for these 2 clock muxes.

Signed-off-by: Grzegorz Jaszczyk <grzegorz.jaszczyk@linaro.org>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>