IOMMU: add common API for device reserved memory

IOMMU: add common API for device reserved memory

Add API similar to rmrr= and ivmd= arguments, but in a common code. This
will allow drivers to register reserved memory regardless of the IOMMU
vendor.
The direct reason for this API is xhci-dbc console driver (aka xue),
that needs to use DMA. But future change may unify command line
arguments for user-supplied reserved memory, and it may be useful for
other drivers in the future too.

This commit just introduces an API, subsequent patches will plug it in
appropriate places. The reserved memory ranges needs to be saved
locally, because at the point when they are collected, Xen doesn't know
yet which IOMMU driver will be used.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Jan Beulich <jbeulich@suse.com>