[FS#1444] Activate CONFIG_ATH9K_HWRNG in Kernel 4.9

[LEDE Bugs]

A new Flyspray task has been opened.  Details are below. 

User who did this - Tom Li (biergaizi) 

Attached to Project - OpenWrt/LEDE Project
Summary - Activate CONFIG_ATH9K_HWRNG in Kernel 4.9
Task Type - Feature Request
Category - Kernel
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Low
Priority - Very Low
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - Since Linux 4.5, ath9k can be used as a random number generator using ADC register as a source of entropy. It is well-known that insufficient entropy is often a problem on routers and weakens security, using ath9k as a random number generator can greatly boost the available entropy on the routers without the need of additional measures such as haveged. 

More information can be found at the following URL:
https://bugs.openwrt.org/index.php?do=details&task_id=1444