riscv: mm: Only compile pgtable.c if MMU

All functions defined in there depend on MMU, so no need to compile it
for !MMU configs.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20231213203001.179237-4-alexghiti@rivosinc.com


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