ARM: dts: am4372: Add the PRU-ICSS0 DT node

ARM: dts: am4372: Add the PRU-ICSS0 DT node

The AM4376+ SoCs have a second smaller PRU-ICSS subsystem (PRUSS0) in
addition to the primary PRUSS1 instance. The PRUSS0 has less DRAM
per PRU, and no Shared DRAM among other minor differences. The IEP
and MII_RT modules even though present within the IP are not pinned
out.

This PRUSS0 instance has a weird SoC integration. It shares the same
L3 OCP interconnect interface with PRUSS1, and also shares its reset
line and clocks. Any external accesses from PRUSS0 requires the PRUSS1's
PRUSS_SYSCFG register to be programmed properly. That said, it is its
own IP instance (a cut-down version), and so it has been added as an
independent node (sibling node to PRUSS1 node) and a child node of the
corresponding PRUSS target module interconnect node. This allows the
PRUSS0 instance to be enabled/disabled independently of the PRUSS1
instance.

The nodes are added under the corresponding interconnect target module
node in the common am4372 dtsi file. The PRU-ICSS instances are not
supported on AM4372 SoC though in the AM437x family, so the interconnect
target module node should be disabled in any derivative board dts file that
uses AM4372 SoCs. The individual PRUSS node can be disabled in the
corresponding board dts file if desired.

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: am437x-pru0_0-fw
     PRU-ICSS0 PRU1 Core: am437x-pru0_1-fw

Note:
1. There are few more sub-modules like the Industrial Ethernet Peripheral
   (IEP), eCAP, UART, that do not have bindings and so will be added in the
   future. Only UART is pinned out, so others should be added in disabled
   state if added.
2. The PRUSS0 INTC on AM437x SoCs routes the host interrupt 5 to the other
   PRUSS1, so it is already marked reserved through the 'ti,irqs-reserved'
   property.

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