nrf5: Fix misuse of flash bank number

nrf5: Fix misuse of flash bank number

Make driver_priv point directly into the corresponding chip bank structure
and add a pointer to it to get back to its chip when it's needed. This
removes the need to keep track of any bank number, either global or chip-
local.

In addition, it simplifies the cases where the chip structure was just used
to access the chip bank fields; now they are directly accessible.

Change-Id: Iaa353cd4fa7d8ff94c2ef69028c7cb32fade0420
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/4775
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>