dt-bindings: soc: ti: pruss: Update IEP documentation

Update the PRUSS documentation to point to IEP documentation.
The examples have also been updated to add the required clocks
property, with the IEP mux clocks added.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
[s-anna@ti.com: Update examples]
Signed-off-by: Suman Anna <s-anna@ti.com>