hpet: clarify maintainer entry

The existing HPET maintainer entries are somewhat unclear about which one
applies to what part of the kernel.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>