ARM: dts: am335x-evm: Enable PRU-ICSS module

The PRU-ICSS target module node was left in disabled state in the
base am33xx-l4.dtsi file. PRU-ICSS is supported on the AM335x EVM,
so enable this node on the AM335x EVM. The PRUSS node and most of
its child nodes are already enabled in the base dts file, and so
become effective automatically with the enabling of this PRU-ICSS
target module node.

The corresponding PRU nodes can be disabled later on if there are
no use-cases defined to use a particular PRU core or the whole
PRU-ICSS subsystem itself if both its PRU cores are unused.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>