cmd: mmc: provide boot area protection command

Provide command 'mmc wp' to power on write protect boot areas on eMMC
devices.

The B_PWR_WP_EN bit in the extended CSD register BOOT_WP is set. The boot
area are write protected until the next power cycle occurs.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/drivers/mmc/mmc.c b/drivers/mmc/mmc.c
index bfcdaa6..fc3123c 100644
--- a/drivers/mmc/mmc.c
+++ b/drivers/mmc/mmc.c
@@ -810,6 +810,11 @@
 	return __mmc_switch(mmc, set, index, value, true);
 }
 
+int mmc_boot_wp(struct mmc *mmc)
+{
+	return mmc_switch(mmc, EXT_CSD_CMD_SET_NORMAL, EXT_CSD_BOOT_WP, 1);
+}
+
 #if !CONFIG_IS_ENABLED(MMC_TINY)
 static int mmc_set_card_speed(struct mmc *mmc, enum bus_mode mode,
 			      bool hsdowngrade)