Move IPI related parts back to (ARM) machine specific file now, when
the interrupt framework is also going to be used by another (MIPS)
architecture. IPI implementations may vary much across different
architectures.

An IPI implementation should still define INTR_IPI_COUNT and use
intr_ipi_setup_counters() to setup IPI counters which are inside of
intrcnt[] and intrnames[] arrays. Those are used for sysctl and ddb.
Then, intr_ipi_increment_count() should be used to increment obtained
counter.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D5459