remoteproc: create a 'recovery' debugfs entry

Add a 'recovery' debugfs entry to dynamically disable/enable recovery
at runtime. This is useful when one is trying to debug an rproc crash;
without it, a recovery will immediately take place, making it harder
to debug the crash.

Contributions from Subramaniam Chanderashekarapuram.

Examples:

- disabling recovery:
$ echo disabled > <debugfs>/remoteproc/remoteproc0/recovery

- in case you want to recover a crash, but keep recovery disabled
  (useful in debugging sessions when you expect additional crashes
   you want to debug):
$ echo recover > <debugfs>/remoteproc/remoteproc0/recovery

- enabling recovery:
$ echo enabled > <debugfs>/remoteproc/remoteproc0/recovery

Signed-off-by: Fernando Guzman Lugo <fernando.lugo@ti.com>
[ohad: some white space, commentary and commit log changes]
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>