drm/i915/gvt: Introduce a framework for tracking HW registers.

This patch introduces a framework for tracking HW registers on different
GEN platforms.

Accesses to GEN HW registers from VMs will be trapped by hypervisor. It
will forward these emulation requests to GVT-g device model, which
requires this framework to search for related register descriptions.

Each MMIO entry in this framework describes a GEN HW registers, e.g.
offset, length, whether it contains RO bits, whether it can be accessed by
LRIs...and also emulation handlers for emulating register reading and
writing.

- Use i915 MMIO register definition & statement.(Joonas)

Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>