flash/nor/nrf5: fix protection setting on nRF51

flash/nor/nrf5: fix protection setting on nRF51

Protection setting has not ever worked. UICR CLENR0 register cannot
be simply written but has to programmed because it resides in UICR
page of the flash.

Enable flash programming before writing CLENR0 and set back to r/o
afterwards.

Inform the user that reset might be required.

Change-Id: Ib0f96c74ba3583ac33f4394ddb57d8c8895adf53
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5586
Tested-by: jenkins