stm32:l4: pwr: Add en/disable_backup_domain_write_protect()

include/libopencm3/stm32/l4/pwr.h

@@ -170,6 +170,8 @@ enum pwr_vos_scale {
 BEGIN_DECLS
 
 void pwr_set_vos_scale(enum pwr_vos_scale scale);
+void pwr_disable_backup_domain_write_protect(void);
+void pwr_enable_backup_domain_write_protect(void);
 
 END_DECLS
 

lib/stm32/l4/pwr.c

@@ -52,4 +52,24 @@ void pwr_set_vos_scale(enum pwr_vos_scale scale)
 	}
 	PWR_CR1 = reg32;
 }
+
+/** Disable Backup Domain Write Protection
+ *
+ * This allows backup domain registers to be changed. These registers are write
+ * protected after a reset.
+ */
+void pwr_disable_backup_domain_write_protect(void)
+{
+	PWR_CR1 |= PWR_CR1_DBP;
+}
+
+/** Re-enable Backup Domain Write Protection
+ *
+ * This protects backup domain registers from inadvertent change.
+ */
+void pwr_enable_backup_domain_write_protect(void)
+{
+	PWR_CR1 &= ~PWR_CR1_DBP;
+}
+
 /**@}*/