target-sparc: implement NPT timer bit

If the NPT bit is set in the timer register, all non-supervisor read accesses
to the register should fail with a privilege exception.

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>
