x86: Introduce and use MP IRQ trigger and polarity defines

MP_IRQDIR_* constants pointed in the right direction but remained unused so
far: It's cleaner to use symbolic values for the IRQ flags in the MP config
table. That also saves some comments.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: jailhouse-dev@googlegroups.com
Link: https://lkml.kernel.org/r/60809926663a1d38e2a5db47d020d6e2e7a70019.1511770314.git.jan.kiszka@siemens.com