remoteproc: k3: Add TI-SCI processor control helper functions

Texas Instruments' K3 generation SoCs have specific modules/register
spaces used for configuring the various aspects of a remote processor.
These include power, reset, boot vector and other configuration features
specific to each compute processor present on the SoC. These registers
are managed by the System Controller such as DMSC on K3 AM65x SoCs.

The Texas Instrument's System Control Interface (TI-SCI) Message Protocol
is used to communicate to the System Controller from various compute
processors to invoke specific services provided by the firmware running
on the System Controller.

Add a common processor control interface header file that can be used by
multiple remoteproc drivers. The helper functions within this header file
abstract the various TI SCI protocol ops for the remoteproc drivers, and
allow them to request the System Controller to be able to program and
manage various remote processors on the SoC. The remoteproc drivers are
expected to manage the life-cycle of their ti_sci_proc_dev local
structures.

Signed-off-by: Suman Anna <s-anna@ti.com>
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20200721223617.20312-4-s-anna@ti.com
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>