irqchip/sifive-plic: Switch to fasteoi flow

The SiFive PLIC interrupt controller seems to have all the HW
features to support the fasteoi flow, but the driver seems to be
stuck in a distant past. Bring it into the 21st century.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Tested-by: Palmer Dabbelt <palmer@sifive.com> (QEMU Boot)
Tested-by: Darius Rad <darius@bluespec.com> (on 2 HW PLIC implementations)
Tested-by: Paul Walmsley <paul.walmsley@sifive.com> (HiFive Unleashed)
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/8636gxskmj.wl-maz@kernel.org