drm/etnaviv: add optional reset support

Add optional reset support which is mentioned in vivante,gc.yaml to
allow the driver to work on SoCs whose reset signal is asserted by default

Signed-off-by: Philippe Lecointre <philippe.lecointre@thalesgroup.com>
Acked-by: Simon Lenain <simon.lenain@thalesgroup.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>