riscv: Introduce support for APLIC interrupt controller

This patch introduces support for the RISC-V APLIC interrupt controller
[1]. Currently, it is only supports direct mode, i.e. without an IMSIC
and functionally replacing the legacy RISC-V PLIC. Work on IMSIC support
is in progress.

[1] https://github.com/riscv/riscv-aia/releases/tag/1.0

Reviewed by: mhorne
Discussed with: jrtc27
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D43293