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/davinci_mmc.c b/drivers/mmc/davinci_mmc.c
index ee8f261..e2379e3 100644
--- a/drivers/mmc/davinci_mmc.c
+++ b/drivers/mmc/davinci_mmc.c
@@ -388,6 +388,7 @@
 	mmc->set_ios = dmmc_set_ios;
 	mmc->init = dmmc_init;
 	mmc->getcd = NULL;
+	mmc->getwp = NULL;
 
 	mmc->f_min = 200000;
 	mmc->f_max = 25000000;