mmc: add support for write protection

Add generic mmc write protection functionality.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
diff --git a/drivers/mmc/gen_atmel_mci.c b/drivers/mmc/gen_atmel_mci.c
index 67b2dbe..70a9f91 100644
--- a/drivers/mmc/gen_atmel_mci.c
+++ b/drivers/mmc/gen_atmel_mci.c
@@ -349,6 +349,7 @@
 	mmc->set_ios = mci_set_ios;
 	mmc->init = mci_init;
 	mmc->getcd = NULL;
+	mmc->getwp = NULL;
 
 	/* need to be able to pass these in on a board by board basis */
 	mmc->voltages = MMC_VDD_32_33 | MMC_VDD_33_34;