ARM: dts: am57xx: Add 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 node and other child nodes. The two PRU-ICSSs
are identical to each other. They are not supported on DRA7xx SoCs in
general, so the nodes are added under the respective interconnect target
module nodes in a common am57-pruss.dtsi file. The file is already
included 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. 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. All the Data
RAMs are represented within a child node of its own named 'memories'
without any compatible. The Real Time Media Independent Interface
controller (MII_RT), and the CFG sub-module are represented as syscon
nodes. The PRUSS CFG module has a clock mux for IEP clock, this clk
node is added under the CFG child node 'clocks'. The default source
for this mux clock is the ICSS_IEP_CLK clock.

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

Note:
1. There are few more sub-modules like the Industrial Ethernet Peripheral
   (IEPs), MDIO, UART, eCAP that do not have bindings and so will be added
   in the future.
2. The PRUSS INTC on AM57xx SoCs also connect the host interrupts 6 and 7
   as possible DMA events, so use the 'ti,irqs-reserved' property in
   derivative board dts files _if_ any of them should not be handled by
   the host OS.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>

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