vfio: platform: reset: Add Broadcom FlexRM reset module

This patch adds Broadcom FlexRM low-level reset for
VFIO platform.

It will do the following:
1. Disable/Deactivate each FlexRM ring
2. Flush each FlexRM ring

The cleanup sequence for FlexRM rings is adapted from
Broadcom FlexRM mailbox driver.

Signed-off-by: Anup Patel <anup.patel@broadcom.com>
Reviewed-by: Oza Oza <oza.oza@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>