ARM: dts: am335x: Update the PRUSS IEP node

Update the compatible for the PRU-ICSS IEP node to ti,am3356-icss-iep, so
that icss_iep driver can be probed. Also add the required clocks property
to IEP node.

Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
[s-anna@ti.com: update iep clock]
Signed-off-by: Suman Anna <s-anna@ti.com>