qcacld-3.0: Add vendor attribute for high RSSI roam trigger threshold

Currently the roam scan high RSSI delta is configured via INI.
Define an attribute to allow user configure high RSSI roam
trigger threshold. STA is expected to trigger roam if the current
connected AP's RSSI gets above this high RSSI threshold. STA's
roam attempt on high RSSI threshold aims to find candidates from
other better Wi-Fi bands.
This attribute value is given priority over the INI.
Use a new service bit WMI_SERVICE_5GHZ_HI_RSSI_ROAM_SUPPORT to
enable high RSSI roam trigger in 5 GHz as well.

Change-Id: Ide48ad2261b603de36bd1b31114b91c3a9d6606f
CRs-Fixed: 3586170