ARM: dts: am33xx-l4: Add PRUSS node

ARM: dts: am33xx-l4: Add PRUSS node

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 a pruss
node and other child nodes. PRU-ICSS is not supported on AM3352 SoC
though in the AM33xx family, so the nodes are added under the
corresponding disabled interconnect target module node in the common
am33xx-l4 dtsi file. The target module node 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. 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 PRU_ICSS_IEP_GCLK 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-ICSS PRU0 Core: am335x-pru1_0-fw
     PRU-ICSS PRU1 Core: am335x-pru1_1-fw

Note:
1. There are few more sub-modules like the Industrial Ethernet Peripheral
   (IEP), MDIO, UART, eCAP that do not have bindings and so will be added
   in the future.
2. The PRUSS INTC on AM335x SoCs also connect the host interrupts 0 to
   TSC_ADC; 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>