dt-bindings: riscv: add sv57 mmu-type

Dumping the dtb from new versions of QEMU warns that sv57 is an
undocumented mmu-type. The kernel has supported sv57 for about a year,
so bring it into the fold.
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230424-rival-habitual-478567c516f0@spudSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Documentation/devicetree/bindings/riscv/cpus.yaml

@@ -65,6 +65,7 @@ properties:
       - riscv,sv32
       - riscv,sv39
       - riscv,sv48
+      - riscv,sv57
       - riscv,none
 
   riscv,cbom-block-size: