staging: most: add new file configfs.c

This patch adds the file configfs.c to the driver directory. The file
registers the necessary subsystems with configfs in order to move the
driver configuration from sysfs to configfs.

Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>