ath9k: export HW random number generator

We measured the FFT-based entropy in 3 ways, Shannon entropy,
collision entropy, and directly measured min-entropy. Just to
be conservative, we recommend the estimated min-Entropy to be
10 bits per 16-bit value.

Analysis was done by Jacobson,David(djacobso@qti.qualcomm.com).

Signed-off-by: Miaoqing Pan <miaoqing@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>