RISC-V: Avoid empty create_*_mapping definitions

These trigger a handful of build warnings.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 677b9eb8 ("riscv: mm: Prepare pt_ops helper functions for sv57")
Link: https://lore.kernel.org/r/20220420184056.7886-2-palmer@rivosinc.com


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