ARM: dts: imx6: RDU2: disallow RMI4 device sleep

This is causing issues with some specific controller configurations, as
the platform isn't power limited it's better to keep it disabled.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>