MAINTAINERS: Add Borislav to the x86 maintainers

Borislav is effectivly maintaining parts of X86 already, make it official.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Borislav Petkov <bp@alien8.de>