ARM: dts: imx6: rdu2: Disable WP for USDHC2 and USDHC3

RDU2 production units come with resistor connecting WP pin to
correpsonding GPIO DNPed for both SD card slots. Drop any WP related
configuration and mark both slots with "disable-wp".

Reported-by: Chris Healy <cphealy@gmail.com>
Reviewed-by: Chris Healy <cphealy@gmail.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Shawn Guo <shawnguo@kernel.org>