dt-bindings: riscv: add a capacity-dmips-mhz cpu property

Since commit 03f11f03db ("RISC-V: Parse cpu topology during boot.")
RISC-V has used the generic arch topology code, which provides for
disparate CPU capacities. We never defined a binding to acquire this
information from the DT though, so document the one already used by the
generic arch topology code: "capacity-dmips-mhz".

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Ley Foon Tan <leyfoon.tan@starfivetech.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230104180513.1379453-3-conor@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Documentation/devicetree/bindings/riscv/cpus.yaml

@@ -114,6 +114,12 @@ properties:
       List of phandles to idle state nodes supported
       by this hart (see ./idle-states.yaml).
 
+  capacity-dmips-mhz:
+    description:
+      u32 value representing CPU capacity (see ../cpu/cpu-capacity.txt) in
+      DMIPS/MHz, relative to highest capacity-dmips-mhz
+      in the system.
+
 required:
   - riscv,isa
   - interrupt-controller