nl80211: Introduce scan flags to emphasize requested scan behavior

This commit defines new scan flags (LOW_SPAN, LOW_POWER, HIGH_LATENCY)
to emphasize the requested scan behavior for the driver. These flags
are optional and are mutually exclusive. The implementation of the
respective functionality can be driver/hardware specific.

These flags can be used to control the compromise between how long
a scan takes, how much power it uses, and high accurate/complete
the scan is in finding the BSSs.

Change-Id: I8f69b89a846da0b5ab89d90e215731bb4f01856c
CRs-Fixed: 2173869
Signed-off-by: Srinivas Dasari <dasaris@codeaurora.org>