bus: introduce bus abstraction

bus: introduce bus abstraction

This patch introduces the rte_bus abstraction for EAL.
The model is:
 - One or more devices are connected to a Bus
 - Drivers are running instances which manage one or more devices
 - Bus is responsible for identifying devices (and interrupt propogation)
 - Driver is responsible for initializing the device

This patch adds a 'rte_bus' base class which would be extended for
specific implementations. It also introduces Bus registration and
deregistration functions.

Signed-off-by: Shreyansh Jain <shreyansh.jain@nxp.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>
Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>