Bluetooth: Poll for RSSI updates from the remote device

This feature is required for implementing Low energy
proximity client immediate alert service. The proximtiy
client registers with the kernel for receiving RSSI updates
of the remote device. The kernel starts polling for RSSI
value using READ_RSSI HCI command and When it reaches
a particular threshold set by the proximity client,
updates are sent to the client.

Change-Id: I723dd140ad3bf02978c57b3ab127bb6768f28cca
CRs-fixed: 344936
Signed-off-by: Archana Ramachandran <archanar@codeaurora.org>