ARM: dts: beagle-x15: Enable PRU-ICSS and the child PRU nodes

The two PRU-ICSS processor subsystems nodes and their associated
child PRU remote processor nodes were left in disabled state in
the base dra7.dts file. The PRU-ICSSs are supported on only the
AM57xx SoCs, and are now enabled in the supported Beagle-X15
board.

These nodes can be disabled later on if there is no use-case
defined to use that remote PRU core or PRU-ICSS subsystem if
both its PRU cores are unused.

Signed-off-by: Suman Anna <s-anna@ti.com>