cnss: Add PCIe power control support

Export a PCIe power control API to WLAN host driver who will decide
which suspend mode (D3 or D0 mode) it will enter based on the number
of connected clients and then call this API to vote PCIe link down/up
accordingly.

CRs-Fixed: 685771
Change-Id: Ib44938b94ba07a1b220615e9f1febcafca8f4277
Signed-off-by: Yuanyuan Liu <yuanliu@codeaurora.org>
Signed-off-by: Yue Ma <yuem@codeaurora.org>