staging: most: add ABI documentation

This patchg adds the sysfs-bus-most.txt file to the source tree.
It is needed to have an ABI description of the driver's sysfs
interface.

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