ath11k: Add support for multibus support

Current design supports only AHB interface for
11ax chipset. Refactor the code by adding hif layer
for bus level abstraction to support  PCI based device.

Signed-off-by: Govind Singh <govinds@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200506094400.4740-2-govinds@codeaurora.org