Allow ACS channel list to be configured as frequencies (in MHz)

Allow ACS channel list to be configured as frequencies (in MHz)

The channel numbers are duplicated between 2.4 GHz / 5 GHz bands and 6
GHz band. Hence, add support to configure a list of frequencies to ACS
(freqlist) instead of a list of channel numbers (chanlist). Also, both 5
GHz and 6 GHz channels are referred by HOSTAPD_MODE_IEEE80211A. The 6
GHz channels alone can be configured by using both mode and frequency
list.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>