Make 'offset' and 'length' parameters optional, if both are omitted
simply read the whole flash bank.

Additionally, check if the 'offset' and 'length' arguments are out of
bounds of the flash bank.

Change-Id: Ib9c1b0538a2c78ebcf702e2da11468dff407f8ff
Signed-off-by: Marc Schink <openocd-dev@marcschink.de>
Reviewed-on: http://openocd.zylin.com/3862


Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>