ath10k: add hif start/stop methods for wcn3990 snoc layer

Add hif start/stop callback for allocating/freeing buffers
on tx/rx pipe and enabling/disabling the tx/rx pipe
interrupts.

Signed-off-by: Govind Singh <govinds@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>