net/mlx5: Introduce vhca state event notifier

vhca state events indicates change in the state of the vhca that may
occur due to a SF allocation, deallocation or enabling/disabling the
SF HCA.

Introduce vhca state event handler which will be used by SF devlink
port manager and SF hardware id allocator in subsequent patches
to act on the event.

This enables single entity to subscribe, query and rearm the event
for a function.

Signed-off-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Vu Pham <vuhuong@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>