flash/nor/tcl: add read_bank and verify_bank

flash/nor/tcl: add read_bank and verify_bank

The only read access to flash chips so is through the target's
memory. Flashes like jtagspi do not expose a memory mapped interface
to the flash. These commands use the flash_driver_read() driver API
directly.

Change-Id: I40b910de650114a3f676507f9f059a234377d862
Signed-off-by: Robert Jordens <jordens@gmail.com>
Reviewed-on: http://openocd.zylin.com/2842
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>