ath10k: make CE layer bus agnostic

Remove bus specific dependencies from CE layer
to have common CE layer across multiple targets.
This is required for adding support for WCN3990
chipset support as WCN3990 chipset uses SNOC
bus interface with Copy Engine endpoint.

Signed-off-by: Govind Singh <govinds@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>