dt-bindings: interrupt-controller: RISC-V local interrupt controller

Add documentation on the RISC-V local interrupt controller, which is a
per-hart interrupt controller that manages all interrupts entering a
RISC-V hart.  This interrupt controller is present on all RISC-V systems.
Signed-off-by: NPalmer Dabbelt <palmer@dabbelt.com>
[hch: minor cleanups]
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NPalmer Dabbelt <palmer@sifive.com>

Documentation/devicetree/bindings/interrupt-controller/riscv,cpu-intc.txt

+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
+controller 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>
+- 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 = "riscv,cpu-intc", "sifive,fu540-c000-cpu-intc";
+			interrupt-controller;
+		};
+	};