ath10k: add initial SDIO support

Chipsets like QCA6584 have support for SDIO so add initial SDIO bus support to
ath10k. With this patch we have the low level HTC protocol working and it's
possible to boot the firmware, but it's still not possible to connect or
anything like. More changes are needed for full functionality. For that reason
we print during initialisation:

WARNING: ath10k SDIO support is incomplete, don't expect anything to work!

Signed-off-by: Erik Stromdahl <erik.stromdahl@gmail.com>
[kvalo@qca.qualcomm.com: refactoring, cleanup, commit log]
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>