nl80211: add common API to configure SAR power limitations

nl80211: add common API to configure SAR power limitations

NL80211_CMD_SET_SAR_SPECS is added to configure SAR from
user space. NL80211_ATTR_SAR_SPEC is used to pass the SAR
power specification when used with NL80211_CMD_SET_SAR_SPECS.

Wireless driver needs to register SAR type, supported frequency
ranges to wiphy, so user space can query it. The index in
frequency range is used to specify which sub band the power
limitation applies to. The SAR type is for compatibility, so later
other SAR mechanism can be implemented without breaking the user
space SAR applications.

Normal process is user space queries the SAR capability, and
gets the index of supported frequency ranges and associates the
power limitation with this index and sends to kernel.

Here is an example of message send to kernel:
8c 00 00 00 08 00 01 00 00 00 00 00 38 00 2b 81
08 00 01 00 00 00 00 00 2c 00 02 80 14 00 00 80
08 00 02 00 00 00 00 00 08 00 01 00 38 00 00 00
14 00 01 80 08 00 02 00 01 00 00 00 08 00 01 00
48 00 00 00

NL80211_CMD_SET_SAR_SPECS:  0x8c
NL80211_ATTR_WIPHY:     0x01(phy idx is 0)
NL80211_ATTR_SAR_SPEC:  0x812b (NLA_NESTED)
NL80211_SAR_ATTR_TYPE:  0x00 (NL80211_SAR_TYPE_POWER)
NL80211_SAR_ATTR_SPECS: 0x8002 (NLA_NESTED)
freq range 0 power: 0x38 in 0.25dbm unit (14dbm)
freq range 1 power: 0x48 in 0.25dbm unit (18dbm)

Signed-off-by: Carl Huang <cjhuang@codeaurora.org>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Abhishek Kumar <kuabhs@chromium.org>
Link: https://lore.kernel.org/r/20201203103728.3034-2-cjhuang@codeaurora.org
[minor edits, NLA parse cleanups]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>