arm64: dts: ti: k3-am65-main: Add ICSSG nodes

Add the DT nodes for the ICSSG0, ICSSG1 and ICSSG2 processor subsystems
that are present on the K3 AM65x SoCs. The three ICSSGs are identical
to each other for the most part, with the ICSSG2 supporting slightly
enhanced features for supporting SGMII PRU Ethernet. Each ICSSG instance
is represented by a PRUSS subsystem node. These nodes are enabled by
default.

The ICSSGs on K3 AM65x SoCs are super-sets of the PRUSS on the AM57xx/
6AK2G SoCs except for larger Shared Data RAM and the lack of a PRU-ICSS
crossbar. They include two auxiliary PRU cores called RTUs and few other
additional sub-modules. The interrupt integration is also different on
the K3 AM65x SoCs and are propagated through various SoC-level Interrupt
Router and Interrupt Aggregator blocks.

The ICSSG subsystem node contains the entire address space. The various
sub-modules of the ICSSG are represented as individual child nodes (so
platform devices themselves) of the PRUSS subsystem node. These include
the two PRU cores and the interrupt controller. The Industrial Ethernet
Peripherals (IEPs), the Real Time Media Independent Interface controller
(MII_RT), and the CFG sub-module are represented as syscon nodes. Each
ICSSG instance has 2 IEP instances and a single MII_G_RT instance. All
the Data RAMs are represented within a child node of its own named
'memories' without any compatible.

The DT nodes use all standard properties. The regs property in the
PRU/RTU nodes define the addresses for the Instruction RAM, the Debug
and Control sub-modules for that PRU core. The firmware for each
PRU/RTU core is defined through a 'firmware-name' property.

The default names for the firmware images for each PRU and RTU core
are defined as follows (these can be adjusted either in derivative
board dts files or through sysfs at runtime if required):
  ICSSG0 PRU0 Core: am65x-pru0_0-fw ; ICSSG0 RTU0 Core: am65x-rtu0_0-fw
  ICSSG0 PRU1 Core: am65x-pru0_1-fw ; ICSSG0 RTU1 Core: am65x-rtu0_1-fw
  ICSSG1 PRU0 Core: am65x-pru1_0-fw ; ICSSG1 RTU0 Core: am65x-rtu1_0-fw
  ICSSG1 PRU1 Core: am65x-pru1_1-fw ; ICSSG1 RTU1 Core: am65x-rtu1_1-fw
  ICSSG2 PRU0 Core: am65x-pru2_0-fw ; ICSSG2 RTU0 Core: am65x-rtu2_0-fw
  ICSSG2 PRU1 Core: am65x-pru2_1-fw ; ICSSG2 RTU1 Core: am65x-rtu2_1-fw

TODO:
Add sub-nodes for additional sub-modules like MII_RT2 etc.

Signed-off-by: Suman Anna <s-anna@ti.com>
[rogerq@ti.com: Add IEP and MII-G-RT nodes]
Signed-off-by: Roger Quadros <rogerq@ti.com>