Bluetooth: Add definitions for MGMT_OP_START_SERVICE_DISCOVERY

This patch adds the opcode and structure for Start Service Discovery
operation.

Signed-off-by: Jakub Pawlowski <jpawlowski@google.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>