MAINTAINERS: Add Mark Gross and Hans de Goede as x86 platform drivers maintainers

Darren Hart and Andy Shevchenko lately have not had enough time to
maintain the x86 platform drivers, dropping their status to:
"Odd Fixes".

Mark Gross and Hans de Goede will take over maintainership of
the x86 platform drivers. Replace Darren and Andy's entries with
theirs and change the status to "Maintained".

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Mark Gross <mgross@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>