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/nand/driver.h

@@ -37,6 +37,9 @@ struct nand_flash_controller
 	/** Driver name that is used to select it from configuration files. */
 	const char *name;
 
+	/** Usage of flash command registration. */
+	const char *usage;
+
     const struct command_registration *commands;
 
 	/** NAND device command called when driver is instantiated during configuration. */