MAINTAINERS: Add myself to X86 PLATFORM DRIVERS as a co-maintainer

For last few months Darren and I are co-maintaining PDx86 subsystem.
Make this fact official by updating MAINTAINERS database.

Acked-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>