SiFive Platform-Level Interrupt Controller (PLIC)
-------------------------------------------------

SiFive SOCs include an implementation of the Platform-Level Interrupt Controller
(PLIC) high-level specification in the RISC-V Privileged Architecture
specification.  The PLIC connects all external interrupts in the system to all
hart contexts in the system, via the external interrupt source in each hart.

A hart context is a privilege mode in a hardware execution thread.  For example,
in an 4 core system with 2-way SMT, you have 8 harts and probably at least two
privilege modes per hart; machine mode and supervisor mode.

Each interrupt can be enabled on per-context basis.  Any context can claim
a pending enabled interrupt and then release it once it has been handled.

Each interrupt has a configurable priority.  Higher priority interrupts are
serviced first.  Each context can specify a priority threshold. Interrupts
with priority below this threshold will not cause the PLIC to raise its
interrupt line leading to the context.

While the PLIC supports both edge-triggered and level-triggered interrupts,
interrupt handlers are oblivious to this distinction and therefore it is not
specified in the PLIC device-tree binding.

While the RISC-V ISA doesn't specify a memory layout for the PLIC, the
"sifive,plic-1.0.0" device is a concrete implementation of the PLIC that
contains a specific memory layout, which is documented in chapter 8 of the
SiFive U5 Coreplex Series Manual <https://static.dev.sifive.com/U54-MC-RVCoreIP.pdf>.

Required properties:
- compatible : "sifive,plic-1.0.0" and a string identifying the actual
  detailed implementation in case that specific bugs need to be worked around.
- #address-cells : should be <0> or more.
- #interrupt-cells : should be <1> or more.
- interrupt-controller : Identifies the node as an interrupt controller.
- reg : Should contain 1 register range (address and length).
- interrupts-extended : Specifies which contexts are connected to the PLIC,
  with "-1" specifying that a context is not present.  Each node pointed
  to should be a riscv,cpu-intc node, which has a riscv node as parent.
- riscv,ndev: Specifies how many external interrupts are supported by
  this controller.

Example:

	plic: interrupt-controller@c000000 {
		#address-cells = <0>;
		#interrupt-cells = <1>;
		compatible = "sifive,plic-1.0.0", "sifive,fu540-c000-plic";
		interrupt-controller;
		interrupts-extended = <
			&cpu0-intc 11
			&cpu1-intc 11 &cpu1-intc 9
			&cpu2-intc 11 &cpu2-intc 9
			&cpu3-intc 11 &cpu3-intc 9
			&cpu4-intc 11 &cpu4-intc 9>;
		reg = <0xc000000 0x4000000>;
		riscv,ndev = <10>;
	};