ath9k_hw: Add hardware code for WoW

add a new file wow.c which takes care of the hardware code
for WoW.

*program the descriptors and data words to periodically
send Keep Alive Frames.
*program the user defined patterns/masks and pattern length
in the hardware registers.
*'ath9k_hw_wow_enable' is called during the drivers suspend
callback which takes care of the following
	- tracking wow event mask (to suppress spurious
	  wow events)
	- properly configure suspend/resume WAR registers
	- configure PCIE PM control register
	- configure MAC WoW registers and their timeouts
	- enabling wow configuration like magic packet,
	  user patterns based on users configuration
	- configuring timeouts for KAL, beacon miss,
	  aifs, slot time, backoff
	- create Keep Alive Pattern ('KAL')
*'ath9k_hw_wow_wakeup' is called during the drivers resume
callback which takes care of the following
	- primary task is to find the reason for wakeup
	  from the wow status register
	- configure/restore AR_PCIE_PM_CTRL register
	- clear all WoW events
	- configure/restore suspend/resume WAR registers

Cc: Senthil Balasubramanian <senthilb@qca.qualcomm.com>
Cc: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Cc: vadivel@qca.qualcomm.com
Signed-off-by: Luis R. Rodriguez <mcgrof@qca.qualcomm.com>
Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>