remoteproc: Change rproc_shutdown() to return a status

The rproc_shutdown() function is currently not returning any
error code, and any failures within rproc_stop() are not passed
back to the users. Change the signature to return a success value
back to the callers.

The remoteproc sysfs and cdev interfaces are also updated to
return back this status to userspace.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220213201246.25952-2-s-anna@ti.com