ath6kl: add system revision support

The driver reads the system_rev value from the kernel,
and loads the appropriate board data file. If an exact match
is not present, the driver will attempt to use the nearest matching
revision before falling back to the original bdata.SD31.bin file.

(cherry picked from commit c095ba1b1fa4f49497622e3a76074a43bba19e8f)

Change-Id: Iaa7957557f3eff804035d5ff6ae9472a1be585fc