drivers/nble: Implement on_nble_sm_security_request_evt()

Implement handling SM Security Requests. Fixes among others following
Bluetooth PTS test cases:

...
1/6   GATT   TC_GAR_CL_BI_05_C   PASS
2/6   GATT   TC_GAR_CL_BI_22_C   PASS
3/6   GATT   TC_GAR_CL_BI_27_C   PASS
4/6   GATT   TC_GAW_CL_BI_06_C   PASS
5/6   GATT   TC_GAW_CL_BI_24_C   PASS
...

Change-Id: Id35004a134bb9898e3b0dd51309f1762676ca579
Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>