intrng: Allow alternative IPI PICs to be registered and used

On RISC-V, the root PIC (whether the PLIC or, as will be the case in
future, the local interrupt controller) cannot send IPIs, relying on
another means to trigger the necessary software interrupts (firmware
calls), but there are upcoming standard devices that will be able to
inject them, so we can't just put the firmware calls in the root PIC
driver.

Thus, split out a new intr_ipi_dev from intr_irq_root_dev to use for
sending IPIs. New devices can be registered with a given priority up
until the first IPI is set up, when the best device seen so far gets
frozen as the IPI device to use.

Reviewed by: mhorne
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D35899