drivers: ieee802154: support Key Identifier Mode > 1

IEEE 802.15.4-2020 defines four possible values for Key Identifier Mode
field of the Auxiliary Security Header. The current ieee802154 driver
API only supports two of them: b00 and b01. This commit adds support for
the two remaining Key Identifier Mode values. It's done by replacing a
field that can only hold Key Index into a field that can holds a pointer
to the entire Key Identifier field.

See IEEE 802.15.4-2020, sections 9.4.2.3 and 9.4.4 for further reference.

Signed-off-by: Jędrzej Ciupis <jedrzej.ciupis@nordicsemi.no>