arch: arm: Use smaller ISR stack size for 16k nRF51 SoCs

The 16k RAM nRF51 variants pose a challenge to get applications to fit
within the available memory. Make the default ISR stack size smaller
than the previous 2k default, but big enough to run fully functional
Bluetooth controller and host stacks.

Change-Id: Ie7c5bb21a3ba620d283e6228a2482d280f85119d
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>