firmware: rsu: document sysfs interface

Describe Intel Stratix10 Remote System Update (RSU) device attributes

Signed-off-by: Richard Gong <richard.gong@intel.com>
Reviewed-by: Alan Tull <atull@kernel.org>
Link: https://lore.kernel.org/r/1567516701-26026-4-git-send-email-richard.gong@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>