dt-bindings: remoteproc: pru: Update bindings for K3 AM65x & J721E SoCs

The K3 AM65x SoCs have the next generation of the PRU-ICSS IP, commonly
called ICSSG. The ICSSG IP on AM65x SoCs has two PRU cores and two
additional custom PRU cores called Real Time Units (RTUs).

The K3 J721E SoCs have a revised version of the ICSSG IP present on K3
AM65x SoCs. The ICSSG IP on K3 J721E SoCs has the same set of two PRU
cores, two RTU cores and a new pair of custom auxiliary PRU cores called
Transmit PRUs (Tx_PRUs).

Update the PRU remoteproc bindings for these PRU cores on these newer
generation ICSSG instances.

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