platform/surface: aggregator: Add platform-drivers-x86 list to MAINTAINERS entry

platform/surface: aggregator: Add platform-drivers-x86 list to MAINTAINERS entry

The Surface System Aggregator Module driver entry is currently missing a
mailing list. Surface platform drivers are discussed on the
platform-driver-x86 list and all other Surface platform drivers have a
reference to that list in their entries. So let's add one here as well.

Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
Link: https://lore.kernel.org/r/20210514221954.5976-1-luzmaximilian@gmail.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>