nrf51: fix UICR region size

nrf51: fix UICR region size

The UICR region is actually 0x100 bytes in size.  Besides making the
full region accessible, having the right value is important because
GDB rounds flash addresses to the nearest multiple of the block size
when determing which flash blocks to erase.

Change-Id: I416c391cbfc7be41a03a9b9c6e42326c87391f38
Signed-off-by: Jim Paris <jim@jtan.com>
Reviewed-on: http://openocd.zylin.com/2361
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Tested-by: jenkins