irqchip: Add RISC-V advanced PLIC driver

The RISC-V advanced interrupt architecture (AIA) specification defines
a new interrupt controller for managing wired interrupts on a RISC-V
platform. This new interrupt controller is referred to as advanced
platform-level interrupt controller (APLIC) which can forward wired
interrupts to CPUs (or HARTs) as local interrupts OR as message
signaled interrupts.
(For more details refer https://github.com/riscv/riscv-aia

)

This patch adds an irqchip driver for RISC-V APLIC found on RISC-V
platforms.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>