ACPI: tables: Allow BGRT to be overridden

Thinkpad T Series expose a malformed BGRT table with Version field set
to 0. This fact prevents bootsplashes (as Plymouth) to correctly show
the manufacturer logo. This patch permits to override malformed BGRT
table with a correct one defined by the user.

Signed-off-by: Andrea Oliveri <oliveriandrea@gmail.com>
[ rjw: Subject ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>