cnss2: Add bus layer

Bus layer is a shim layer for potential buses (PCIe, USB and SDIO)
of WLAN devices. It abstracts all the bus related APIs for common
files of the driver so that it can help make them bus independent.

Change-Id: I06e19a26d34168fe0fcc65229a4519b868cd97c1
Signed-off-by: Yue Ma <yuem@codeaurora.org>