microbit: Update of maximum BLE bond count from 2 to 4.

BLE bond information is stored in a table in flash memory during the pairing process with another BLE device.
This information is required for a secure BLE connection.

There is a limit to the number of entries in this table. It used to be 4 but was recently changed to 2
during a code merge with mbed updates. This patch returns to support four bonds.

source/nordic_sdk/components/ble/device_manager/config/device_manager_cnfg.h

@@ -85,7 +85,7 @@
  *       be stored. In such cases, application will be notified with DM_DEVICE_CONTEXT_FULL 
  *       as event result at the completion of the security procedure.
  */
-#define DEVICE_MANAGER_MAX_BONDS         2
+#define DEVICE_MANAGER_MAX_BONDS         4
 
 
 /**