flash: introduce .usage field for nand and nor flash driver structure

Change-Id: I47e7ec8fa8c70d2addc3aa52d7c97e9e1e7bb662
Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>
Reviewed-on: http://openocd.zylin.com/301
Tested-by: jenkins
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

src/flash/nor/driver.h

@@ -56,6 +56,11 @@ struct flash_driver
 	 */
 	const char *name;
 
+	/**
+	 * Gives a human-readable description of arguments.
+	 */
+	const char *usage;
+
 	/**
 	 * An array of driver-specific commands to register.  When called
 	 * during the "flash bank" command, the driver can register addition