ARM: dts: DRA7: Add the PRU-ICSS nodes

ARM: dts: DRA7: Add the PRU-ICSS nodes

Add the DT nodes for the PRU-ICSS1 and PRU-ICSS2 processor subsystems
that are present on AM57xx family of SoCs. Each PRU-ICSS instance is
represented by a pruss-soc-bus node and a child PRUSS subsystem node.
The two PRU-ICSSs are identical to each other. They are not supported
on DRA7xx SoCs in general, so the nodes are added in disabled state
to the common dra7 DTS file. They should be enabled only in the AM57xx
related board files.

The PRU-ICSSs on AM57xx are very similar to the PRUSS in AM33xx and
AM437x except for variations in the RAM sizes and the number of
interrupts coming into the MPU INTC. The interrupt events into the
PRU-ICSS also requires programming of the corresponding crossbars
properly.

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-ICSS1 PRU0 Core: am57xx-pru1_0-fw
    PRU-ICSS1 PRU1 Core: am57xx-pru1_1-fw
    PRU-ICSS2 PRU0 Core: am57xx-pru2_0-fw
    PRU-ICSS2 PRU1 Core: am57xx-pru2_1-fw

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