sun4u: split NPT and INT_DIS accesses between timer and compare registers

Accesses to the timer register high bit should only set NPT, whilst accesses
to the timer compare register high bit should only set INT_DIS. This fixes
issues with the timer being unexpectedly disabled whilst trying to boot
FreeBSD SPARC64.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-By: Artyom Tarasenko <atar4qemu@gmail.com>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
