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
nrf51: fix UICR erase

nrf51_erase_page() checks for (sector->offset == NRF51_UICR_BASE) to
determine if the UICR should be erased.  However, sector->offset for
the UICR bank is set to 0 in nrf51_probe, so this code is never hit.
Attempting to erase UICR ends up erasing the first flash sector.

Use bank->base instead to determine if UICR is being erased.

Change-Id: Ie5df0f9732f23662085ae2b713d64968cd801472
Signed-off-by: Jim Paris <jim@jtan.com>
Reviewed-on: http://openocd.zylin.com/2362
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>