dt-bindings: riscv: cpus: Clarify mmu-type interpretation

The current description implies that only a single address translation
mode is available to the operating system. However, some implementations
support multiple address translation modes, and the operating system is
free to choose between them.

Per the RISC-V privileged specification, Sv48 implementations must also
implement Sv39, and likewise Sv57 implies support for Sv48. This means
it is possible to describe all supported address translation modes using
a single value, by naming the largest supported mode. This appears to
have been the intended usage of the property, so note it explicitly.

Fixes: 4fd669a8 ("dt-bindings: riscv: convert cpu binding to json-schema")
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20231227175739.1453782-1-samuel.holland@sifive.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Documentation/devicetree/bindings/riscv/cpus.yaml

@@ -63,8 +63,8 @@ properties:
 
   mmu-type:
     description:
-      Identifies the MMU address translation mode used on this
-      hart.  These values originate from the RISC-V Privileged
+      Identifies the largest MMU address translation mode supported by
+      this hart.  These values originate from the RISC-V Privileged
       Specification document, available from
       https://riscv.org/specifications/
     $ref: /schemas/types.yaml#/definitions/string