RISC-V Hart-Level Interrupt Controller (HLIC)
---------------------------------------------

RISC-V cores include Control Status Registers (CSRs) which are local to each
CPU core (HART in RISC-V terminology) and can be read or written by software.
Some of these CSRs are used to control local interrupts connected to the core.
Every interrupt is ultimately routed through a hart's HLIC before it
interrupts that hart.

The RISC-V supervisor ISA manual specifies three interrupt sources that are
attached to every HLIC: software interrupts, the timer interrupt, and external
interrupts.  Software interrupts are used to send IPIs between cores.  The
timer interrupt comes from an architecturally mandated real-time timer that is
controlled via Supervisor Binary Interface (SBI) calls and CSR reads.  External
interrupts connect all other device interrupts to the HLIC, which are routed
via the platform-level interrupt controller (PLIC).

All RISC-V systems that conform to the supervisor ISA specification are
required to have a HLIC with these three interrupt sources present.  Since the
interrupt map is defined by the ISA it's not listed in the HLIC's device tree
entry, though external interrupt controllers (like the PLIC, for example) will
need to define how their interrupts map to the relevant HLICs.  This means
a PLIC interrupt property will typically list the HLICs for all present HARTs
in the system.

Required properties:
- compatible : "riscv,cpu-intc"
- #interrupt-cells : should be <1>.  The interrupt sources are defined by the
  RISC-V supervisor ISA manual, with only the following three interrupts being
  defined for supervisor mode:
    - Source 1 is the supervisor software interrupt, which can be sent by an SBI
      call and is reserved for use by software.
    - Source 5 is the supervisor timer interrupt, which can be configured by
      SBI calls and implements a one-shot timer.
    - Source 9 is the supervisor external interrupt, which chains to all other
      device interrupts.
- interrupt-controller : Identifies the node as an interrupt controller

Furthermore, this interrupt-controller MUST be embedded inside the cpu
definition of the hart whose CSRs control these local interrupts.

An example device tree entry for a HLIC is show below.

	cpu1: cpu@1 {
		compatible = "riscv";
		...
		cpu1-intc: interrupt-controller {
			#interrupt-cells = <1>;
			compatible = "sifive,fu540-c000-cpu-intc", "riscv,cpu-intc";
			interrupt-controller;
		};
	};