ath6kl: query device tree for firmware board-id

When no default board data file is present query the device tree for a
board-id setting to identify the board data to use.  If the FDT lacks the
necesary info fall back to the previous behaviour of using a compile-time
board filename.

Signed-off-by: Sam Leffler <sleffler@chromium.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>