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 PRUs do not have a unified address space (have an Instruction
RAM and a primary Data RAM at both 0x0). The PRU remoteproc driver
therefore uses a custom remoteproc core ELF loader ops. The added
.da_to_va ops is only used to provide translations for the PRU
Data RAMs. 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 for each PRU core, 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 driver currently supports the AM335x, AM437x, AM57xx and 66AK2G
SoCs, and support for other TI SoCs will be added in subsequent
patches.

Co-developed-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>
Co-developed-by: Grzegorz Jaszczyk <grzegorz.jaszczyk@linaro.org>
Signed-off-by: Grzegorz Jaszczyk <grzegorz.jaszczyk@linaro.org>
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20201208141002.17777-3-grzegorz.jaszczyk@linaro.org
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

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