Generalize IPI support for ARM intrng and use it for interrupt
controller IPI provider.

New struct intr_ipi is defined which keeps all info about an IPI:
its name, counter, send and dispatch methods. Generic intr_ipi_setup(),
intr_ipi_send() and intr_ipi_dispatch() functions are implemented.

An IPI provider must implement two functions:
(1) an intr_ipi_send_t function which is able to send an IPI,
(2) a setup function which initializes itself for an IPI and

calls intr_ipi_setup() with appropriate arguments.

Differential Revision: https://reviews.freebsd.org/D5700