ARM: dts: AM33xx: Add the PRU-ICSS DT nodes

ARM: dts: AM33xx: Add the PRU-ICSS DT nodes

Add the DT nodes for the PRU-ICSS on AM33xx family of SoCs. The
AM33xx SoCs contain a single PRU-ICSS instance and is represented
by the pruss-soc-bus node and a child PRUSS node. PRU-ICSS is not
supported on AM3352 SoC though in the AM33xx family, so the nodes
are added in disabled state to the common am33xx dtsi file. They
should be enabled in only those derivative board files that use
a SoC containing PRU-ICSS.

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-ICSS PRU0 Core: am335x-pru1_0-fw
     PRU-ICSS PRU1 Core: am335x-pru1_1-fw

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