flash/nor: consolidate flash protect/protect_check

Make flash_driver methods protect() and protect_check()
optional.

Remove dummy definitions of these methods from the drivers
which do not implement protection handling.

Some drivers did not define protect method. It raised segfault
before this change and now it is handled properly.

Lot of drivers returned ERROR_OK from dummy protect()
- now flash_driver_protect() returns an error if not handled by the driver.

Change-Id: I2d4a0da316bf03c6379791b1b1c6198fbf22e66c
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4765
Tested-by: jenkins
Reviewed-by: Andreas Bolsch <hyphen0break@gmail.com>
Reviewed-by: Christopher Head <chead@zaber.com>

src/flash/nor/driver.h

@@ -109,6 +109,8 @@ struct flash_driver {
 	/**
 	 * Bank/sector protection routine (target-specific).
 	 *
+	 * If protection is not implemented, set method to NULL
+	 *
 	 * When called, the driver should enable/disable protection
 	 * for MINIMUM the range covered by first..last sectors
 	 * inclusive. Some chips have alignment requirements will
@@ -178,6 +180,8 @@ struct flash_driver {
 	 * flash_sector_s::is_protected field for each of the flash
 	 * bank's sectors.
 	 *
+	 * If protection is not implemented, set method to NULL
+	 *
 	 * @param bank - the bank to check
 	 * @returns ERROR_OK if successful; otherwise, an error code.
 	 */