arch-riscv: RISC-V HiFive Platform implementation

This patch implements the RISC-V HiFive Platform
(based on the SiFive HiFive series). The HiFive
platform requires a PLIC and CLINT device (based on
current HiFive boards).

The optional uart_int_id field redirects Console interrupts
to the PLIC interrupt controller.

PlicDevice is a wrapper around BasicPioDevice that
requires an interrupt ID. Interrupts are expected to
be raised via platform->postPciInterrupt(id).

VirtIOMMIO is a slight modification from the ARM
implementation such that interrupts are posted through
PLIC

PlicDevice is a wrapper around BasicPioDevice that
requires an interrupt ID. Interrupts are expected to
be raised via platform->postPciInterrupt(id).

VirtIOMMIO is a slight modification from the ARM
implementation such that interrupts are posted through
PLIC.

Uart8250 was modified slightly for easier Python
setup.

Change-Id: I4bbdb5f903b52a41d1b7e0ccc44877c46cd30d56
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40599
Reviewed-by: Ayaz Akram <yazakram@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
12 files changed