cnss: Add support for PCIe link auto suspend feature

When there is no active traffic to WLAN firmware, PCIe link can be
suspended. In the current design, PCIe link goes to L2 state only
when kernel suspends. However if WLAN driver can detect the inactivity
at the bus, it can put the PCIe link to L2 when the link is not in
active use. Export APIs that can be used by WLAN driver to autonomously
control the PCIe link state.

CRs-Fixed: 761959
Change-Id: I9da1eac3763f85ac51b1e8f7c921cfebd05c2672
Signed-off-by: Sameer Thalappil <sameert@codeaurora.org>