mtd: spi-nor: Add documentation for spi_nor_soft_reset()

Document what the function does and that it should only be used when it
is known that the device supports it. This will avoid unaware
programmers thinking that they can arbitrarily use it to reset the
device.

Suggested-by: Michael Walle <michael@walle.cc>
Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Michael Walle <michael@walle.cc>