Documentation: dt: soc: Add description for knav qmss driver

* Texas Instruments Keystone Navigator Queue Management SubSystem driver

Driver source code path

  drivers/soc/ti/knav_qmss.c

  drivers/soc/ti/knav_qmss_acc.c

The QMSS (Queue Manager Sub System) found on Keystone SOCs is one of

the main hardware sub system which forms the backbone of the Keystone

multi-core Navigator. QMSS consist of queue managers, packed-data structure

processors(PDSP), linking RAM, descriptor pools and infrastructure

Packet DMA.

The Queue Manager is a hardware module that is responsible for accelerating

management of the packet queues. Packets are queued/de-queued by writing or

reading descriptor address to a particular memory mapped location. The PDSPs

perform QMSS related functions like accumulation, QoS, or event management.

Linking RAM registers are used to link the descriptors which are stored in

descriptor RAM. Descriptor RAM is configurable as internal or external memory.

The QMSS driver manages the PDSP setups, linking RAM regions,

queue pool management (allocation, push, pop and notify) and descriptor

pool management.

knav qmss driver provides a set of APIs to drivers to open/close qmss queues,

allocate descriptor pools, map the descriptors, push/pop to queues etc. For

details of the available APIs, please refers to include/linux/soc/ti/knav_qmss.h