power: reset: remove unused imx-snvs-poweroff driver

There's no user of it in kernel now and it basically functions the same
as the generic syscon-poweroff.c to which we have already switched.
So let's remove it.

Cc: Robin Gong <yibin.gong@nxp.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>