VFIO: platform: Calxeda xgmac reset module

This patch introduces a module that registers and implements a basic
reset function for the Calxeda xgmac device. This latter basically disables
interrupts and stops DMA transfers.

The reset function code is inherited from the native calxeda xgmac driver.

Signed-off-by: Eric Auger <eric.auger@linaro.org>
Acked-by: Baptiste Reynal <b.reynal@virtualopensystems.com>
Tested-by: Baptiste Reynal <b.reynal@virtualopensystems.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>