Wifi: Support check on device capability for 6GHZ

This commit adds the HAL APIs and implementation to support checking
whether device supports 6GHz band or not.
VTS test has been added for the new api.

Bug: 139354972
Test: Manual: Make sure capability is read correctly.
Test: VTS test.
Change-Id: I03b0c6017666ffd3663d5979d6e6a2bd93043c48