mlxsw: pci: Add support for performing bus reset

This is a preparation stage before introducing hot reload. During the
reload process the ASIC should be resetted by accessing the PCI BAR due
to unavailability of the mailbox/emad interfaces.

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>