ath6kl: add softmac support

Change-Id: I0693c76dc677bd7ae19b81277ceb1c62f2bb87f0
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.

Change-Id: I3122016bb439e98b2a58f682a4e83815e9174d8d