boards/arduino_101: defconfig: Enable nRF51 PM if NBLE is selected

To avoid explicitly having to state this in every application's config
file simply set the defaults to the sanest values.

Change-Id: I2c2bbd2424a12ec9a36bbd6d0c9cd9a1d259e5e5
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>