bindings: soc: ti: add documentation for k3 ringacc

* Texas Instruments K3 NavigatorSS Ring Accelerator

The Ring Accelerator (RA) is a machine which converts read/write accesses

from/to a constant address into corresponding read/write accesses from/to a

circular data structure in memory. The RA eliminates the need for each DMA

controller which needs to access ring elements from having to know the current

state of the ring (base address, current offset). The DMA controller

performs a read or write access to a specific address range (which maps to the

source interface on the RA) and the RA replaces the address for the transaction

with a new address which corresponds to the head or tail element of the ring

(head for reads, tail for writes).

The Ring Accelerator is a hardware module that is responsible for accelerating

management of the packet queues. The K3 SoCs can have more than one RA instances

Required properties:

- compatible	: Must be "ti,am654-navss-ringacc";

- reg		: Should contain register location and length of the following

		  named register regions.

- reg-names	: should be

		  "rt" - The RA Ring Real-time Control/Status Registers

		  "fifos" - The RA Queues Registers

		  "proxy_gcfg" - The RA Proxy Global Config Registers

		  "proxy_target" - The RA Proxy Datapath Registers

- ti,num-rings	: Number of rings supported by RA

- ti,sci-rm-range-gp-rings : TI-SCI RM subtype for GP ring range

- ti,sci	: phandle on TI-SCI compatible System controller node

- ti,sci-dev-id	: TI-SCI device id of the ring accelerator

- msi-parent	: phandle for "ti,sci-inta" interrupt controller

Optional properties:

 -- ti,dma-ring-reset-quirk : enable ringacc / udma ring state interoperability

		  issue software w/a

Example:

ringacc: ringacc@3c000000 {

	compatible = "ti,am654-navss-ringacc";

	reg =	<0x0 0x3c000000 0x0 0x400000>,

		<0x0 0x38000000 0x0 0x400000>,

		<0x0 0x31120000 0x0 0x100>,

		<0x0 0x33000000 0x0 0x40000>;

	reg-names = "rt", "fifos",

		    "proxy_gcfg", "proxy_target";

	ti,num-rings = <818>;

	ti,sci-rm-range-gp-rings = <0x2>; /* GP ring range */

	ti,dma-ring-reset-quirk;

	ti,sci = <&dmsc>;

	ti,sci-dev-id = <187>;

	msi-parent = <&inta_main_udmass>;

};

client:

dma_ipx: dma_ipx@<addr> {

	...

	ti,ringacc = <&ringacc>;

	...

}