ARM: dts: dra7: Add PRU system events for virtio

Two PRU system events "vring" and "kick" have been added to each
PRU node in each of the PRU-ICSS1 and PRU-ICSS2 remote processor
subsystems to enable the virtio/rpmsg communication between MPU
and that PRU core. The additions are done in the common base
dra7.dtsi file, and so are inherited by all the AM57xx boards.
Do note that PRUSS is not available/supported on the DRA7xx SoCs
SoCs and is only limited to the AM57xx SoCs.

The PRU system events is the preferred approach over using OMAP
mailboxes, as it eliminates an external peripheral access from
the PRU-side, and keeps the interrupt generation internal to the
PRUSS. The difference from MPU would be minimal in using one
versus the other.

Mailboxes can still be used if desired. Either approach would
require that an appropriate firmware image is loaded/booted on
the PRU.

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