Enhance the interrupt capabilities of ti_pruss driver.

The existing ti_pruss driver for the PRUSS Hardware provided by the AM335x
ARM CPU has basic interrupt capabilities. This updated driver provides some
more options:

• Sysctl based configuration for the interrupts (for some examples, see the test plan in the phabricator review cited below).
• A device file (/dev/pruss0.irqN) for each enabled interrupt. This file can be read and the device blocks if no irq has happened or returns an uint64_t timestamp based on nanouptime().
• Each interrupt device file provides kqueue-based event notification, blocking read(), or select().

Submitted by: Manuel Stuhn <freebsdnewbie@freenet.de>
Differential Revision: https://reviews.freebsd.org/D11959