riscv: Annotate pgtable_l{4,5}_enabled with __ro_after_init

pgtable_l{4,5}_enabled are read only after initialization, make explicit
annotation of __ro_after_init on them.

Signed-off-by: Dawei Li <dawei.li@shingroup.cn>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240320064712.442579-3-dawei.li@shingroup.cn


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