dt-bindings: riscv,cpu-intc: Cleanups from a missed review

I managed to miss one of Rob's code reviews on the mailing list
<http://lists.infradead.org/pipermail/linux-riscv/2018-August/001139.html>.
The patch has already been merged, so I'm submitting a fixup.

Sorry!

Fixes: b67bc7cb ("dt-bindings: interrupt-controller: RISC-V local interrupt controller")
Cc: Rob Herring <robh@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Karsten Merker <merker@debian.org>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

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

@@ -11,7 +11,7 @@ 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
+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).
 
@@ -25,7 +25,15 @@ in the system.
 
 Required properties:
 - compatible : "riscv,cpu-intc"
-- #interrupt-cells : should be <1>
+- #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
@@ -38,7 +46,7 @@ An example device tree entry for a HLIC is show below.
 		...
 		cpu1-intc: interrupt-controller {
 			#interrupt-cells = <1>;
-			compatible = "riscv,cpu-intc", "sifive,fu540-c000-cpu-intc";
+			compatible = "sifive,fu540-c000-cpu-intc", "riscv,cpu-intc";
 			interrupt-controller;
 		};
 	};