flash/nor: add flash mdw/h/b commands

Some flash banks are not mapped in the target memory
(e.g. SPI flash, some special pages).

Add flash version of mdw/h/b which reads data using
the flash driver.

Change-Id: I66910e0a69cf523fe5ca1ed6ce7b9e8e176aef4a
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4776
Tested-by: jenkins
Reviewed-by: Andreas Bolsch <hyphen0break@gmail.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>