MAINTAINERS: Update platform-drivers-x86 maintainer and tree

Update the general entry for drivers/platform/x86 with myself as
maintainer and point to my tree.

Leave Matthew Garrett as maintainer of the two drivers called out
specifically elsewhere in MAINTAINERS.

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>