remoteproc/pru: Add support for various PRU cores on K3 J721E SoCs

The K3 J721E family of SoCs have a revised version of the AM65x ICSSG IP
and contains two instances of this newer ICSSG IP. Each ICSSG processor
subsystem contains 2 primary PRU cores, 2 auxiliary PRU cores called RTUs,
and 2 new auxiliary cores called Transmit PRUs (Tx_PRUs).

Each RTU and Tx_PRU cores have their 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, while the Tx_PRU cores is
normally used to control the TX L2 FIFO if enabled in Ethernet
applications. Both can also be used to run independent applications.
The RTU and Tx_PRU 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 cores have the same
memory copying limitations as on AM65x, so reuses the custom memcpy
function within the driver's ELF loader implementation. The initial
names for the firmware images for each PRU core are retrieved from
DT nodes, and can be adjusted through sysfs if required.

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