ARM: dts: keystone-k2g: Add the PRU-ICSS nodes

ARM: dts: keystone-k2g: Add the PRU-ICSS nodes

Add the DT nodes for the PRU-ICSS0 and PRU-ICSS1 processor subsystems
that are present on the 66AK2G SoC. The two PRU-ICSSs are identical
to each other. Each PRU-ICSS instance is represented by a pruss-soc-bus
node and a child PRUSS subsystem node. These nodes are enabled by
default.

The PRU-ICSSs on 66AK2G are very similar to the PRUSS on the AM57xx
SoCs except for larger Shared Data RAM and the lack of a PRU-ICSS
crossbar. There are also few other minor integration differences
w.r.t IPC mechansims that can be attributed to the architecture
differences between Keystone and OMAP families.

The PRUSS subsystem node contains the entire address space and the
various interrupts generated towards the main MPU. The various
sub-modules of the PRU-ICSS 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 Peripheral (IEP), the Real Time Media Independent
Interface controller (MII_RT), and the CFG sub-module are represented
as syscon nodes. 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 nodes define the addresses for the Instruction RAM, the Debug
and Control sub-modules for that PRU core. The firmware for each
PRU core is defined through a 'firmware-name' property.

The default names for the firmware images for each PRU core are
defined as follows (these can be adjusted either in derivative
board dts files or through sysfs at runtime if required):
    PRU-ICSS0 PRU0 Core: k2g-pru0_0-fw
    PRU-ICSS0 PRU1 Core: k2g-pru0_1-fw
    PRU-ICSS1 PRU0 Core: k2g-pru1_0-fw
    PRU-ICSS1 PRU1 Core: k2g-pru1_1-fw

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