drivers/nble: Add timeout before Conn Param Update as Peripheral

According to Core 4.2 Vol 3, Part C, 9.3.12.2,
The Peripheral device should not perform a Connection Parameter
Update procedure within T_GAP (conn_pause_peripheral) after
establishing a connection.

T_GAP (conn_pause_peripheral) is 5 seconds.
Fixes TC_LE_REJ_BI_01_C.

1/3   L2CAP   TC_LE_REJ_BI_01_C   PASS
2/3   L2CAP   TC_LE_CPU_BI_02_C   PASS
3/3   L2CAP   TC_LE_CPU_BV_01_C   PASS

1/3   GAP   TC_CONN_CPUP_BV_01_C   PASS
2/3   GAP   TC_CONN_CPUP_BV_02_C   PASS
3/3   GAP   TC_CONN_CPUP_BV_03_C   PASS

Change-Id: I66bf57a2b8323b748c15f61b2daecfaa435dbb69
Signed-off-by: Mariusz Skamra <mariusz.skamra@tieto.com>