remoteproc/pru: Add support for various PRU cores on K3 AM65x SoCs

The K3 AM65x family of SoCs have the next generation of the PRU-ICSS
processor subsystem, commonly referred to as ICSSG. Each ICSSG processor
subsystem contain two primary PRU cores and two new auxiliary PRU cores
called RTUs. Each RTU core has its own dedicated IRAM (smaller than a
PRU), Control and debug feature sets, but is different in terms of
sub-modules integrated around it and does not have the full capabilities
associated with a PRU core. The RTU core is typically used to aid a
PRU core in accelerating data transfers, but can also be used to run
independent applications. The RTU cores though share the same Data RAMs
as the PRU cores, so the memories have to be partitioned carefully
between different applications. The new cores also support a new
sub-module called Task Manager to support two different context thread
executions.

Enhance the existing PRU remoteproc driver to support these new PRU
and RTU cores by using specific compatibles. The initial names for the
firmware images for each PRU core are retrieved from DT nodes, and can
be adjusted through sysfs if required.

The PRU remoteproc driver has to be specifically modified to use a
custom memcpy function within its ELF loader implementation for these
new cores in order to overcome a limitation with copying data into each
of the core's IRAM memories. These memory ports support only 4-byte
writes, and any sub-word order byte writes clear out the remaining
bytes other than the bytes being written within the containing word.
The default ARM64 memcpy also cannot be used as it throws an exception
when the preferred 8-byte copy operation is attempted. This choice is
made by using a state flag that is set only on K3 SoCs. This can be
reworked to use match data to scale beyond current SoCs if needed.

Signed-off-by: Suman Anna <s-anna@ti.com>