remoteproc: add rproc_report_crash function to notify rproc crashes

Allow low-level remoteproc drivers to report rproc crashes by exporting
a new rproc_report_crash() function (invoking this from non-rproc drivers
is probably wrong, and should be carefully scrutinized if ever needed).

rproc_report_crash() can be called from any context; it offloads the
tasks of handling the crash to a separate thread.

Handling the crash from a separate thread is helpful because:
- Ability to call invoke rproc_report_crash() from atomic context, due to
  the fact that many crashes trigger an interrupt, so this function can be
  called directly from ISR context.
- Avoiding deadlocks which could happen if rproc_report_crash() is called
  from a function which indirectly holds the rproc lock.

Handling the crash might involve:
- Remoteproc register dump
- Remoteproc stack dump
- Remoteproc core dump
- Saving Remoteproc traces so they can be read after the crash
- Reseting the remoteproc in order to make it functional again ...