net/mlx5: Introduce a new header file for physical port functions

BugLink: http://bugs.launchpad.net/bugs/1528466



All the device physical port access functions are implemented in the
port.c file.
We just extract the exposure of these functions from driver.h into a
dedicated header file called port.h.

Signed-off-by: Achiad Shochat <achiad@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(back ported from commit ada68c31)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

 Conflicts:
	drivers/infiniband/hw/mlx5/main.c

Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>