hpet: x86_64 and i386 are not different code any more

hpet implementation for x86_64 and i386 is unified. Reflect
this in MAINTAINERS.

Acked-by: Venkatesh Pallipadi <venki@google.com>
Acked-by: Vojtech Pavlik <vojtech@suse.cz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>