Add a few #defines for the class 0, 1 and 2 interrupt status bits, and
use them instead of magic numbers when we're setting or checking for
these interrupts.

Also, add a #define for the class 2 mailbox threshold interrupt mask.
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>