riscv: mm: unexport _pgtable_l4_enabled and _pgtable_l5_enabled

Now, these two vars are only used by kernel itself, especially the code
before MMU on. No modules users any more, unexport them.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Link: https://patch.msgid.link/20261008150045.28905-6-jszhang@kernel.org
Signed-off-by: Paul Walmsley <pjw@kernel.org>