ARM: dts: am335x-evmsk: 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 SK
EVM board, so enable this node to support PRUSS on this board. 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>