Use the standard name for the gpion in DT: reset-gpios

Document that the RST line is low active and update the example
accordingly.
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20191120131753.6831-2-peter.ujfalusi@ti.comSigned-off-by: Mark Brown <broonie@kernel.org>