RISC-V: mm: Update pgtable comment documentation

sv57 is supported in the kernel so pgtable.h should reflect that.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20230809232218.849726-4-charlie@rivosinc.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>