flash/nor/nrf5: fix allocation of driver_priv and sector array

flash/nor/nrf5: fix allocation of driver_priv and sector array

Drop static pointer to allocated struct nrf5_info, iterate over
the flash bank list to find previously allocated nrf5 instances.
nrf5 is swd only device, so static allocation makes no harm,
but we should avoid copying the wrong code to other flash drivers.

Free sector array before allocating it to avoid memory leak on
re-probing device.

Change-Id: I781d8f4418a91c043f2393e5ecc5278fc6df3566
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4910
Tested-by: jenkins