remoteproc/pru: Add a PRU remoteproc driver

remoteproc/pru: Add a PRU remoteproc driver

The Programmable Real-Time Unit Subsystem (PRUSS) consists of
dual 32-bit RISC cores (Programmable Real-Time Units, or PRUs)
for program execution. This patch adds a remoteproc platform
driver for managing the individual PRU RISC cores life cycle.

The PRU remoteproc driver uses the standard remoteproc core ELF
loader. However, the PRUs do not have a unified address space,
(has an Instruction RAM and a primary Data RAM at both 0x0) and
leverage an added .da_to_va ops to use the standard ELF loader.
This remoteproc driver does not have support for error recovery
and system suspend/resume features. Different compatibles are
used to allow providing scalability for instance-specific device
data if needed. The driver uses a default firmware-name retrieved
from device-tree, and the firmwares are expected to be present
in the standard Linux firmware search paths. They can also be
adjusted by userspace if required through the sysfs interface
provided by the remoteproc core.

The PRU remoteproc driver uses a client-driven boot methodology
- it does _not_ support auto-boot so that the PRU load and boot
is dictated by the corresponding client drivers for achieving
various usecases. This allows flexibility for the client drivers
or applications to set a firmware name (if needed) based on their
desired functionality and boot the PRU. The sysfs bind and unbind
attributes have also been suppressed so that the PRU devices cannot
be unbound and thereby shutdown a PRU from underneath a PRU client
driver.

The PRU interrupt configuration is handled within the PRUSS INTC
platform driver, and leverages the system events to interrupt
channels and host interrupts mapping configuration through a
PRU firmware resource table for now. This will be revisited and
enhanced in the future for a better interface. The mappings
are currently programmed during the booting/shutdown of the PRU.

The driver currently supports the AM335x SoC, and support for other
TI SoCs will be added in subsequent patches.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Andrew F. Davis <afd@ti.com>