MAINTAINERS: Add Andy and Darren as arch/x86/platform/ reviewers

... so that they can get CCed on platform patches.

Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Darren Hart <dvhart@infradead.org>
Cc: Andy Shevchenko <andy@infradead.org>
Cc: x86@kernel.org
Link: https://lkml.kernel.org/r/20190128113619.19025-1-bp@alien8.de