powerpc/pseries: EOI interrupts in XICS by setting lowest priority

Discussing with Benjamin Herrenschmidt, OPAL_INT_GET_XIRR masks the
returned priority, so must be resumed before more interrupts can be
handled at this priority. Since there are only two priorities used in
FreeBSD, we know that the previous priority in an EOI will always be
0xff (lowest priority).

Reviewed by: nwhitehorn
Approved by: re(rgrimes)
Differential Revision: https://reviews.freebsd.org/D17361