mmc: sdhci-esdhc-imx: add write protect on custom GPIO on mx25/35

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Tested-by: Marc Reilly <marc@cpdesign.com.au>
Tested-by: Eric Benard <eric@eukrea.com>
Signed-off-by: Chris Ball <cjb@laptop.org>