stm class: Clarify configfs root type/operations names

The current naming of stp-policy root type and group ops is confusing,
rename them for better readability.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Tested-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>